[x86] Teach my pshufb comment printer to handle VPSHUFB forms as well as
PSHUFB forms. This will be important to update some AVX tests when I add
PSHUFB combining.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214624 91177308-0d34-0410-b5e6-96231b3b80d8