[TargetLowering] Improve SimplifyDemandedVectorElts/SimplifyDemandedBits support

For bitcast nodes from larger element types, add the ability for SimplifyDemandedVectorElts to call SimplifyDemandedBits by merging the elts mask to a bits mask.

I've raised https://bugs.llvm.org/show_bug.cgi?id=39689 to deal with the few places where SimplifyDemandedBits's lack of vector handling is a problem.

Differential Revision: https://reviews.llvm.org/D54679

llvm-svn: 347301