Propagate fmf in IRTranslate for fneg

Summary: This case is related to D63405 in that we need to be propagating FMF on negates.

Reviewers: volkan, spatel, arsenm

Reviewed By: arsenm

Subscribers: wdng, javed.absar

Differential Revision: https://reviews.llvm.org/D63458

llvm-svn: 363631