Utilize new SDNode flag functionality to expand current support for fma

Summary: This patch originated from D47388 and is a proper subset of the originating changes, containing only the fmf optimization guard extensions.

Reviewers: spatel, hfinkel, wristow, arsenm, javed.absar, rampitec, nhaehnle, nemanjai

Reviewed By: rampitec, nhaehnle

Subscribers: tpr, nemanjai, wdng

Differential Revision: https://reviews.llvm.org/D47918

llvm-svn: 334876