Fix issues with ISD::FNEG and ISD::FMA SDNodes where they would not be constant-folded
during DAGCombine in certain circumstances.  Unfortunately, the circumstances required
to trigger the issue seem to require a pretty specific interaction of DAGCombines,
and I haven't been able to find a testcase that reproduces on X86, ARM, or AArch64.
The functionality added here is replicated in essentially every other DAG combine,
so it seems pretty obviously correct.