RE: Popcount optimization using AVX512

[Devulapalli, Raghuveer]

> Thanks for the feedback.  I've attached an updated patch.

(1) Shouldn't it be: return (_xgetbv(0) & 0xe6) == 0xe6; ? Otherwise 
zmm_regs_available() will return false. 
(2) Nitpick: avx512_popcnt_available and avx512_bw_available() run the same 
cpuid leaf. You could combine them into one to avoid running cpuid twice. My 
apologies, I should have mentioned this before.