[SelectionDAG] Add SimplifyDemandedBits to SimplifyDemandedVectorElts simplification

This patch enables SimplifyDemandedBits to call SimplifyDemandedVectorElts in cases where the demanded bits mask covers entire elements of a bitcasted source vector.

There are a couple of cases here where simplification at a deeper level (such as through bitcasts) prevents further simplification - CommitTargetLoweringOpt only adds immediate uses/users back to the worklist when we might want to combine the original caller again to see what else it can simplify.

As well as that I had to disable handling of bool vector until SimplifyDemandedVectorElts better supports some of their opcodes (SETCC, shifts etc.).

Fixes PR39178

Differential Revision: https://reviews.llvm.org/D52935

llvm-svn: 343913