[ARM] Add scheduling classes for VFNM[AS]

The VFNM[AS] instructions did not have scheduling information attached, which
was causing assertion failures with the Cortex-A57 scheduling model and
-fp-contract=fast, because the Cortex-A57 sched model claims to be complete.

Differential Revision: https://reviews.llvm.org/D34139

llvm-svn: 305288