[SPARC] Clean up the support for disabling fsmuld and fmuls instructions.

Summary:
Also enable no-fsmuld for sparcv7 (which doesn't have the
instruction).

The previous code which used a post-processing pass to do this was
unnecessary; disabling the instruction is entirely sufficient.

Reviewers: jacob_hansen, ekedaigle

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35576

llvm-svn: 308661