[X86] Teach X86TargetLowering::targetShrinkDemandedConstant to set non-demanded bits...

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@327530 91177308-0d34-0410-b5e6-96231b3b80d8