[InstCombine] allow SimplifyDemandedVectorElts to work with FP binops

We're a long way from D50992 and D51553, but this is where we have to start.
We weren't back-propagating undefs into binop constant values for anything but
add/sub/mul/and/or/xor. 

This is likely because we have to be careful about not introducing UB/poison 
with div/rem/shift. But I suspect we already are getting the poison part wrong 
for add/sub/mul (although it may not be possible to expose the bug currently
because we use SimplifyDemandedVectorElts from a limited set of opcodes).
See the discussion/implementation from D48987 and D49047.

This patch just enables functionality for FP ops because those do not have 
UB/poison potential.

llvm-svn: 343727