[InstCombine] use m_APInt to allow icmp X, C folds for splat constant vectors

Of course, we really need to refactor and fix all of the cmp predicates, 
but this one is interesting because without it, we later perform an 
information-losing transform of icmp (shl 1, Y), C, and we can't recover
the better fold.

llvm-svn: 279263