Utilize new SDNode flag functionality to expand current support for fsub

Summary: This patch originated from D46562 and is a proper subset, with some issues addressed for fsub.

Reviewers: spatel, hfinkel, wristow, arsenm

Reviewed By: spatel

Subscribers: wdng

Differential Revision: https://reviews.llvm.org/D47910

llvm-svn: 334306