[x86] add folds for x86 vector compare nodes (PR27924)

Ideally, we can get rid of most x86 LLVM intrinsics by transforming them to IR (and some of that happened 
with http://reviews.llvm.org/rL272807), but it doesn't cost much to have some simple folds in the backend
too while we're working on that and as a backstop.

This fixes:
https://llvm.org/bugs/show_bug.cgi?id=27924

Differential Revision: http://reviews.llvm.org/D21356

llvm-svn: 272828