[DAGCombine] ((A-c1)+c2) -> (A+(c2-c1)) constant-fold

Summary:
This was the root cause of the endless combine loop in D62257

https://rise4fun.com/Alive/d3W

Reviewers: RKSimon, spatel, craig.topper, t.p.northover

Reviewed By: t.p.northover

Subscribers: t.p.northover, javed.absar, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62662

llvm-svn: 362131