[X86] Add an override of targetShrinkDemandedConstant to limit the damage that...

[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

llvm-svn: 323048