[DAGCombiner] fold (addcarry (xor a, -1), b, c) -> (subcarry b, a, !c) and flip carry.

Summary:
As per title. DAGCombiner only mathes the special case where b = 0, this patches extends the pattern to match any value of b.

Depends on D57302

Reviewers: hfinkel, RKSimon, craig.topper

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59208

llvm-svn: 366214