Third attempt - still not as good as still having branches

Levenshtein.FullBranchlessV3.cs

		/// <summary>
		/// Using SSE4.1, calculates the costs for the virtual matrix.
		/// This performs a 4x outer loop unrolling allowing fewer lookups of target character and deletion cost data across the rows.
		/// </summary>
		/// <param name="previousRowPtr"></param>
		/// <param name="source"></param>
		/// <param name="rowIndex"></param>
		/// <param name="targetPtr"></param>
		/// <param name="targetLength"></param>
		private static unsafe void CalculateRows_4Rows_Sse41(int* previousRowPtr, ReadOnlySpan<char> source, ref int rowIndex, char* targetPtr, int targetLength)
		{
			var acceptableRowCount = source.Length - 3;

			Vector128<int> row1Costs, row2Costs, row3Costs, row4Costs, row5Costs;
			Vector128<int> sourceChars;

			var allOnesVector = Vector128.Create(1);

			fixed (char* sourcePtr = source)
			{
				var sourceUShortPtr = (ushort*)sourcePtr;
				var targetUShortPtr = (ushort*)targetPtr;

				for (; rowIndex < acceptableRowCount; rowIndex += 4)
				{
					sourceChars = Sse41.ConvertToVector128Int32(Sse3.LoadDquVector128(sourceUShortPtr));
					row1Costs = Vector128.Create(rowIndex); //Sub
					row2Costs = Sse2.Add(row1Costs, allOnesVector); //Insert, Sub
					row3Costs = Sse2.Add(row2Costs, allOnesVector); //Insert, Sub
					row4Costs = Sse2.Add(row3Costs, allOnesVector); //Insert, Sub
					row5Costs = Sse2.Add(row4Costs, allOnesVector); //Insert

					var columnIndex = 0;
					var rowColumnsRemaining = targetLength;

					Vector128<int> targetChar;

					while (rowColumnsRemaining >= 8)
					{
						rowColumnsRemaining -= 8;
						targetChar = Sse41.ConvertToVector128Int32(Sse3.LoadDquVector128(targetUShortPtr + columnIndex));
						CalculateColumn_4Rows_Sse41(ref targetChar, previousRowPtr, ref row1Costs, ref row2Costs, ref row3Costs, ref row4Costs, ref row5Costs, ref allOnesVector, sourceChars, ref columnIndex);
						CalculateColumn_4Rows_Sse41(ref targetChar, previousRowPtr, ref row1Costs, ref row2Costs, ref row3Costs, ref row4Costs, ref row5Costs, ref allOnesVector, sourceChars, ref columnIndex);
						CalculateColumn_4Rows_Sse41(ref targetChar, previousRowPtr, ref row1Costs, ref row2Costs, ref row3Costs, ref row4Costs, ref row5Costs, ref allOnesVector, sourceChars, ref columnIndex);
						CalculateColumn_4Rows_Sse41(ref targetChar, previousRowPtr, ref row1Costs, ref row2Costs, ref row3Costs, ref row4Costs, ref row5Costs, ref allOnesVector, sourceChars, ref columnIndex);
						targetChar = Sse41.ConvertToVector128Int32(Sse3.LoadDquVector128(targetUShortPtr + columnIndex));
						CalculateColumn_4Rows_Sse41(ref targetChar, previousRowPtr, ref row1Costs, ref row2Costs, ref row3Costs, ref row4Costs, ref row5Costs, ref allOnesVector, sourceChars, ref columnIndex);
						CalculateColumn_4Rows_Sse41(ref targetChar, previousRowPtr, ref row1Costs, ref row2Costs, ref row3Costs, ref row4Costs, ref row5Costs, ref allOnesVector, sourceChars, ref columnIndex);
						CalculateColumn_4Rows_Sse41(ref targetChar, previousRowPtr, ref row1Costs, ref row2Costs, ref row3Costs, ref row4Costs, ref row5Costs, ref allOnesVector, sourceChars, ref columnIndex);
						CalculateColumn_4Rows_Sse41(ref targetChar, previousRowPtr, ref row1Costs, ref row2Costs, ref row3Costs, ref row4Costs, ref row5Costs, ref allOnesVector, sourceChars, ref columnIndex);
					}

					if (rowColumnsRemaining >= 4)
					{
						rowColumnsRemaining -= 4;
						targetChar = Sse41.ConvertToVector128Int32(Sse3.LoadDquVector128(targetUShortPtr + columnIndex));
						CalculateColumn_4Rows_Sse41(ref targetChar, previousRowPtr, ref row1Costs, ref row2Costs, ref row3Costs, ref row4Costs, ref row5Costs, ref allOnesVector, sourceChars, ref columnIndex);
						CalculateColumn_4Rows_Sse41(ref targetChar, previousRowPtr, ref row1Costs, ref row2Costs, ref row3Costs, ref row4Costs, ref row5Costs, ref allOnesVector, sourceChars, ref columnIndex);
						CalculateColumn_4Rows_Sse41(ref targetChar, previousRowPtr, ref row1Costs, ref row2Costs, ref row3Costs, ref row4Costs, ref row5Costs, ref allOnesVector, sourceChars, ref columnIndex);
						CalculateColumn_4Rows_Sse41(ref targetChar, previousRowPtr, ref row1Costs, ref row2Costs, ref row3Costs, ref row4Costs, ref row5Costs, ref allOnesVector, sourceChars, ref columnIndex);
					}

					while (rowColumnsRemaining > 0)
					{
						rowColumnsRemaining--;
						targetChar = Sse41.ConvertToVector128Int32(Vector128.Create(targetUShortPtr[columnIndex]));
						CalculateColumn_4Rows_Sse41(ref targetChar, previousRowPtr, ref row1Costs, ref row2Costs, ref row3Costs, ref row4Costs, ref row5Costs, ref allOnesVector, sourceChars, ref columnIndex);
					}
				}
			}
		}

		/// <summary>
		/// Using SSE4.1, calculates the cost for 4 vertically adjacent cells in the virtual matrix.
		/// Comparing 4 vertically adjacent cells prevents 3 target character lookups, 3 deletion cost lookups and 3 saves of the deletion cost.
		/// SSE4.1 instructions allow a virtually branchless minimum value computation when the source and target characters don't match.
		/// </summary>
		/// <param name="targetChar"></param>
		/// <param name="previousRowPtr"></param>
		/// <param name="row1Costs"></param>
		/// <param name="row2Costs"></param>
		/// <param name="row3Costs"></param>
		/// <param name="row4Costs"></param>
		/// <param name="row5Costs"></param>
		/// <param name="allOnesVector"></param>
		/// <param name="columnIndex"></param>
		[MethodImpl(MethodImplOptions.AggressiveInlining)]
		private static unsafe void CalculateColumn_4Rows_Sse41(
			ref Vector128<int> targetChar,
			int* previousRowPtr,
			ref Vector128<int> row1Costs,
			ref Vector128<int> row2Costs,
			ref Vector128<int> row3Costs,
			ref Vector128<int> row4Costs,
			ref Vector128<int> row5Costs,
			ref Vector128<int> allOnesVector,
			Vector128<int> sourceChars,
			ref int columnIndex
		)
		{
			var lastDeletionCost = Vector128.Create(previousRowPtr[columnIndex]);

			var comparisonMask = Sse2.CompareEqual(targetChar, sourceChars);
			var localEqual = Sse2.And(comparisonMask, row1Costs);
			var localNotEqual = Sse2.AndNot(
				comparisonMask, 
				Sse2.Add(
					Sse41.Min(
						Sse41.Min(
							row2Costs,
							row1Costs
						),
						lastDeletionCost
					),
					allOnesVector
				)
			);
			row1Costs = lastDeletionCost;
			lastDeletionCost = Sse2.Or(localEqual, localNotEqual);

			sourceChars = Sse2.ShiftRightLogical128BitLane(sourceChars, 4);

			comparisonMask = Sse2.CompareEqual(targetChar, sourceChars);
			localEqual = Sse2.And(comparisonMask, row2Costs);
			localNotEqual = Sse2.AndNot(
				comparisonMask,
				Sse2.Add(
					Sse41.Min(
						Sse41.Min(
							row3Costs,
							row2Costs
						),
						lastDeletionCost
					),
					allOnesVector
				)
			);
			row2Costs = lastDeletionCost;
			lastDeletionCost = Sse2.Or(localEqual, localNotEqual);

			sourceChars = Sse2.ShiftRightLogical128BitLane(sourceChars, 4);

			comparisonMask = Sse2.CompareEqual(targetChar, sourceChars);
			localEqual = Sse2.And(comparisonMask, row3Costs);
			localNotEqual = Sse2.AndNot(
				comparisonMask,
				Sse2.Add(
					Sse41.Min(
						Sse41.Min(
							row4Costs,
							row3Costs
						),
						lastDeletionCost
					),
					allOnesVector
				)
			);
			row3Costs = lastDeletionCost;
			lastDeletionCost = Sse2.Or(localEqual, localNotEqual);

			sourceChars = Sse2.ShiftRightLogical128BitLane(sourceChars, 4);

			comparisonMask = Sse2.CompareEqual(targetChar, sourceChars);
			localEqual = Sse2.And(comparisonMask, row4Costs);
			localNotEqual = Sse2.AndNot(
				comparisonMask,
				Sse2.Add(
					Sse41.Min(
						Sse41.Min(
							row5Costs,
							row4Costs
						),
						lastDeletionCost
					),
					allOnesVector
				)
			);
			row4Costs = lastDeletionCost;
			row5Costs = Sse2.Or(localEqual, localNotEqual);

			previousRowPtr[columnIndex++] = row5Costs.GetElement(0);

			targetChar = Sse2.ShiftRightLogical128BitLane(targetChar, 4);
		}