[X86][AVX] Use Zeroable mask to improve shuffle mask widening

Noticed while updating D42044, lowerV2X128VectorShuffle can improve the shuffle mask with the zeroable data to create a target shuffle mask to recognise more 'zero upper 128' patterns.

NOTE: lowerV4X128VectorShuffle could benefit as well but the code needs refactoring first to discriminate between SM_SentinelUndef and SM_SentinelZero for negative shuffle indices.

Differential Revision: https://reviews.llvm.org/D49092

llvm-svn: 336900