[DAGCombine] Use FoldConstantArithmetic() to perform C2-(A+C1) -> (C2-C1)-A fold

Summary:
No tests change, and i'm not sure how to test this, but it's better safe than sorry.

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

Reviewed By: craig.topper

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62663

llvm-svn: 362132