[X86] Teach lowerShuffleAsBlend to use bit blend for v16i8/v32i8/v16i16 when...

[X86] Teach lowerShuffleAsBlend to use bit blend for v16i8/v32i8/v16i16 when avx512vl is enabled but not avx512bw.

Probably not super important since there are no real CPUs with
avx512vl and not avx512bw. But vpternlog should be better than
vblendvb.

I do wonder if we should use vpternlog even with BWI. We
currently use vblendmb or vpblendmw by putting the mask into a GPR
and moving it to a k-register. But I don't think we hoist the
GPR to k-register copy in machine LICM. Using VPTERNLOG would use
a constant pool load, but has the advantage that we're pretty good
at hoisting and rematerializing those.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D83156