[mips] Generate NMADD and NMSUB instructions when fneg node is present

This patch enables generation of NMADD and NMSUB instructions when fneg node
is present. These instructions are currently only generated if fsub node is
present.

Patch by Stanislav Ocovaj.

Differential Revision: https://reviews.llvm.org/D34507

llvm-svn: 311862