JIT: Initial FPRF support

Doesn't support all the FPSCR flags, just the FPRF ones.

Attempt to optimize FPRF calculation out if the next instruction would
overwrite it (basic peephole optimization).

Currently matches the previous behavior; only do FPRF for fmul and fmadd.

This is a little bit of a WIP as I'm not sure the "right" thing to do here in terms of where we support FPRF. Doing the calculation after every single float op, especially without more aggressive flag-calculation-removal optimization than just basic peepholing, would probably result in a significant performance decrease. But we probably want to at least have it as an option so that we can test if any games *do* need it?