UnreachableCode.cs

var tree = CSharpSyntaxTree.ParseText(@"
    class C
    {
        void M(int x)
        {
            return;
            if(x == 0)                                  //-+     Start is unreachable
                System.Console.WriteLine(""Hello"");    // |
            L1:                                            //-+    End is unreachable
        }
    }
");

var Mscorlib = PortableExecutableReference.CreateFromAssembly(typeof(object).Assembly);
var compilation = CSharpCompilation.Create("MyCompilation",
    syntaxTrees: new[] { tree }, references: new[] { Mscorlib });
var model = compilation.GetSemanticModel(tree);

//Choose first and last statements
var firstIf = tree.GetRoot().DescendantNodes().OfType<IfStatementSyntax>().Single();
var label1 = tree.GetRoot().DescendantNodes().OfType<LabeledStatementSyntax>().Single();

ControlFlowAnalysis result = model.AnalyzeControlFlow(firstIf, label1);
Console.WriteLine(result.StartPointIsReachable);    //False
Console.WriteLine(result.EndPointIsReachable);      //False