[DAGCombiner] fold (add (add (xor a, -1), b), 1) -> (sub b, a)

Summary: This pattern is sometime created after legalization.

Reviewers: efriedma, spatel, RKSimon, zvi, bkramer

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58874

llvm-svn: 355716