[SelectionDAG] allow FP binops in SimplifyDemandedVectorElts

This is intended to make the backend on par with functionality that was 
added to the IR version of SimplifyDemandedVectorElts in:
rL343727
...and the original motivation is that we need to improve demanded-vector-elements 
in several ways to avoid problems that would be exposed in D51553.

Differential Revision: https://reviews.llvm.org/D52912


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344541 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed