[X86] Make use of vpmovwb when possible in LowerMULH

If we have BWI, we can truncate in a much simpler way by using vpmovwb. This even works without VLX by using the wider zmm->ymm truncate with a subvector extract.

Differential Revision: https://reviews.llvm.org/D38375

llvm-svn: 314457