[x86] Teach the vector shuffle yet another step of canonicalization.

No functionality changed yet, but this will prevent subsequent patches
from having to handle permutations of various interleaved shuffle
patterns.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222614 91177308-0d34-0410-b5e6-96231b3b80d8