[IR] remove fake binop query for fneg

We want to remove this fneg API because it would silently fail 
if we add an actual fneg instruction to IR (as proposed in 
D53877 ).

We have a newer 'match' API that makes checking for
these patterns simpler. It also works with vectors
that may include undef elements in constants.

If any out-of-tree users need updating, they can model
their code changes on this commit:
https://reviews.llvm.org/rL345295

llvm-svn: 345904