[InstCombine] refactor fmul with negated op folds; NFCI

The existing code was inefficiently looking for 'nsz' variants.
That's unnecessary because we canonicalize those to the expected
form with -0.0.

We may also want to adjust or remove the fold that sinks negation.
We don't do that for fdiv (or integer ops?). That should be uniform?
It may also lead to missed optimization as in PR21914:
https://bugs.llvm.org/show_bug.cgi?id=21914
...or we just have to fix other passes to avoid that problem.

llvm-svn: 325924