[PATCH] D23781: Fix SystemZ hang caused by r279105

[Elliot Colp via llvm-commits]

colpell created this revision.
colpell added a reviewer: uweigand.
colpell added subscribers: jonpa, llvm-commits.

The change in r279105 causes an infinite loop in some cases, as it sets the upper bits of an AND mask constant, which DAGCombiner::SimplifyDemandedBits then unsets.
This patch reverts that part of the behaviour, instead relying on .td peepholes to perform the transformation to NILL. I reapplied my original fix for the problem addressed by r279105 (unsetting the upper bits, which prevents a compiler abort for a different reason).

https://reviews.llvm.org/D23781

Files:
  lib/Target/SystemZ/SystemZISelLowering.cpp
  lib/Target/SystemZ/SystemZInstrInfo.td

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