[X86] Teach X86TargetLowering::targetShrinkDemandedConstant to set...

[X86] Teach X86TargetLowering::targetShrinkDemandedConstant to set non-demanded bits if it helps created an and mask that can be matched as a zero extend.

I had to modify the bswap recognition to allow unshrunk masks to make this work.

Fixes PR36689.

Differential Revision: https://reviews.llvm.org/D44442

llvm-svn: 327530