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[DAGCombine][X86][AArch64][AMDGPU] (x - y) + -1 -> add (xor y, -1), x fold

Authored by lebedev.ri on May 28 2019, 10:54 AM.

Description

[DAGCombine][X86][AArch64][AMDGPU] (x - y) + -1 -> add (xor y, -1), x fold

Summary:
This prevents regressions in next patch,
and somewhat recovers from the regression to AMDGPU test in D62223.

It is indeed not great that we leave vector decrement,
don't transform it into vector add all-ones..

https://rise4fun.com/Alive/ZRl

Reviewers: RKSimon, craig.topper, spatel, arsenm

Reviewed By: RKSimon, arsenm

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, javed.absar, dstuttard, tpr, t-tye, kristof.beyls, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62263

llvm-svn: 361855