[TargetLowering] Add DemandedElts mask to SimplifyDemandedBits (PR40000)

This is an initial patch to add the necessary support for a DemandedElts argument to SimplifyDemandedBits, more closely matching computeKnownBits and to help improve vector codegen.

I've added only a small amount of the changes necessary to get at least one test to update - a lot more can be done but I'd like to add these methodically with proper test coverage, at the same time the hope is to slowly move some/all of SimplifyDemandedVectorElts into SimplifyDemandedBits as well.

Differential Revision: https://reviews.llvm.org/D55768

llvm-svn: 349374