[X86] Use XOR for SUB (C, X) during isel if will help fold an immediate

Summary:
Same idea as D48529, but restricted to X86 and done very late to avoid any surprises where subtract might be better for DAG combining.

This seems like the safest way to do this trick. And we consider doing it as a DAG combine later.

Reviewers: spatel, RKSimon

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D48557

llvm-svn: 335575