[DAGCombiner] Leverage add's commutativity

Summary: This avoid the need to duplicate all pattern and actually end up exposing some opportunity to optimize existing pattern that did not exists in both directions on an existing test case.

Reviewers: mkuper, spatel, bkramer, RKSimon, zvi

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29541

llvm-svn: 294125