MSVC with AVX2 compile option, C4244 warnings.

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.

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.