[X86] Add support for "light" AVX

AVX/AVX512 instructions may cause frequency drop on e.g. Skylake.
The magnitude of frequency/performance drop depends on instruction
(multiplication vs load/store) and vector width. Currently users,
that want to avoid this drop can specify -mprefer-vector-width=128.
However this also prevents generations of 256-bit wide instructions,
that have no associated frequency drop (mainly load/stores).

Add a tuning flag that allows generations of 256-bit AVX load/stores,
even when -mprefer-vector-width=128 is set, to speed-up memcpy&co.
Verified that running memcpy loop on all cores has no frequency impact
and zero CORE_POWER:LVL[12]_TURBO_LICENSE perf counters.

Makes coping memory faster e.g.:
BM_memcpy_aligned/256 80.7GB/s ± 3% 96.3GB/s ± 9% +19.33% (p=0.000 n=9+9)

Differential Revision: https://reviews.llvm.org/D134982