MSVC with AVX2 compile option, C4244 warnings.

[ghost]

If build with AVX2 instruction set, MSVC has dozens of C4244 warnings:
warning C4244: 'return': conversion from 'unsigned __int64' to 'unsigned int', possible loss of data

MEM_STATIC unsigned ZSTD_countTrailingZeros64(U64 val)
{
    assert(val != 0);
#   if defined(_MSC_VER) && defined(_WIN64)
#       if STATIC_BMI2 == 1
            return _tzcnt_u64(val);
#       else

Adding (unsinged) before _tzcnt_u64(val) should fixes this issue.

[Cyan4973]

Maybe we should also update our Windows CI tests,
so that they can catch such issues.

[ghost]

Maybe we should also update our Windows CI tests,

IMO no need, these codes will not be changed for a long time.

Maybe refactor the architecture-related code into one file, such as clz/ctz/memcpy_16bytes code, this is handy for porting to new architecture.
PR #2831 tried to refactor clz/ctz code, it removes some similar code that spread everywhere.