extend folding fsub/fadd to fneg for FMF

Summary: This change provides a common optimization path for both Unsafe and FMF driven optimization for this fsub fold adding reassociation, as it the flag that most closely represents the translation

Reviewers: spatel, wristow, arsenm

Reviewed By: spatel

Subscribers: wdng

Differential Revision: https://reviews.llvm.org/D50195

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339357 91177308-0d34-0410-b5e6-96231b3b80d8