<xutility>: vectorize std::count

[AlexGuteniev]

Relates to #2379

For contiguous ranges, simple types (1,2,4,8 byte integers, maybe also 4,8 bytes float in fast mode) the following vector algorithm is possible (assuming SSE2 and 8-bit type, but applicable to other sizes/vector sizes):

Spread the value to a vector register (_mm_set1 intrinsics)
Obtain matched bitmask (_mm_cmpeq_epi8 intrinsic)
Get mask as bits (_mm_movemask_epi8) , add them up (_popcnt)
Accumulate this result.
Probably hand-coded popcount will be inefficient, in this case can apply starting SSE4.2, for which we assume popcnt available.

[AlexGuteniev]

#include <algorithm>
#include <array>

char s[] = "the quick brown fox jumps over the lazy dog";

int foxes()
{
    return std::count(std::begin(s), std::end(s), 'o');
}

https://godbolt.org/z/6KeYT4YaG
clang uses my proposal in the generated code
gcc does something which I don't understand, but like less (edit: figured out, yes, it is stupid vectorization, though could beat unvectorized)
MSVC currently naively counts bytes, so library optimization is indeed helpful