[x86] try harder to lower to PCMPGT instead of not-of-PCMPEQ

This is motivated by the example in https://llvm.org/PR50055 ,
but it doesn't do anything for that bug currently because we
don't actually have a zero-extended setcc there.

Proof for the generic transform (inverse of what we would
try to do in combining):
https://alive2.llvm.org/ce/z/aBL-Mg

Differential Revision: https://reviews.llvm.org/D102275