[X86] Call SimplifyDemandedBits on conditions of X86ISD::SHRUNKBLEND

This extends to combineVSelectToShrunkBlend to be able to resimplify SHRUNKBLENDS that have already been created.

This should help some of the regressions from D56387

Differential Revision: https://reviews.llvm.org/D56421

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350875 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed