Fix SystemZ hang caused by r279105

Fix SystemZ hang caused by r279105

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).

Differential Revision: https://reviews.llvm.org/D23781

llvm-svn: 279515