[X86][AVX] Prefer VPBLENDW+VPBLENDD to VPBLENDVB for v16i16 blend shuffles

Noticed while looking at D49562 codegen - we can avoid a large constant mask load and a slow VPBLENDVB select op by using VPBLENDW+VPBLENDD instead.

TODO: As discussed on the patch, we should investigate adding VPBLENDVB handling to target shuffle combining as well, that will allow us to extend this to VPBLENDW+VPBLENDW+VPBLENDD.

Differential Revision: https://reviews.llvm.org/D50074

llvm-svn: 340913