[X86] Lower ISD::UADDO to use the Z flag instead of C flag when the RHS is a constant...

[X86] Lower ISD::UADDO to use the Z flag instead of C flag when the RHS is a constant 1 to encourage INC formation.

Summary:
Add an additional combine to combineCarryThroughADD to reverse it back to the C flag to avoid regressions.

I believe this catches the cases that D57547 got.

Reviewers: RKSimon, spatel

Reviewed By: spatel

Subscribers: javed.absar, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57637

llvm-svn: 352984