[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

llvm-svn: 344541