[X86] Combine vpmovdw+vpacksswb into vpmovdb.

This is similar to the combine we already have for vpmovdw+vpackuswb.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@348910 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed