[x86] use PMOVMSK for vector-sized equality comparisons

We could do better by splitting any oversized type into whatever vector size the target supports, 
but I left that for future work if it ever comes up. The motivating case is memcmp() calls on 16-byte
structs, so I think we can wire that up with a TLI hook that feeds into this.

Differential Revision: https://reviews.llvm.org/D31156

llvm-svn: 298376