[X86] Add an override of targetShrinkDemandedConstant to limit the damage that shrinkdemandedbits can do to zext_in_reg operations
Summary:
This patch adds an implementation of targetShrinkDemandedConstant that tries to keep shrinkdemandedbits from removing bits that would otherwise have been recognized as a movzx.
We still need a follow patch to stop moving ands across srl if the and could be represented as a movzx before the shift but not after. I think this should help with some of the cases that D42088 ended up removing during isel.
Reviewers: spatel, RKSimon
Reviewed By: spatel
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D42265
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@323048
91177308-0d34-0410-b5e6-
96231b3b80d8
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