constexpr u64 mul/add, avoiding overflow warnings on VC++

constexpr_u64.cpp

constexpr uint64_t lo(uint64_t x) { return x & UINT64_C(0xffffffff); }
constexpr uint64_t hi(uint64_t x) { return x >> 32; }

constexpr uint64_t mulu64(uint64_t a, uint64_t b)
{
	return lo(lo(a) * lo(b))
		+ (lo(hi(lo(a) * lo(b))
			+ lo(a) * hi(b)
			+ hi(a) * lo(b)) << 32);
}

constexpr uint64_t addu64(uint64_t a, uint64_t b)
{
	return lo(lo(a) + lo(b))
		+ (lo(hi(lo(a) + lo(b)) + (hi(a) + hi(b))) << 32);
}