[DAGCombine] (A-C1)-C2 -> A-(C1+C2) constant-fold

Summary: https://rise4fun.com/Alive/Mb1M

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

Reviewed By: t.p.northover

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62689

llvm-svn: 362134