JIT64: clean up and unify float load/store code

While we're at it, support a bunch of float load/store variants that weren't
implemented in the JIT. Might not have a big speed impact on typical games but
they're used at least a bit in povray and luabench.

694 -> 644 seconds on povray.
JIT64: add mulhwx implementation
x64Emitter: add support for some missing CVT instructions
Merge pull request #894 from FioraAeterna/missingcvt

x64Emitter: add support for some missing CVT instructions
JIT: make fprf conditional in fcmp, just like the other instructions

Missed in the FPRF merge (it didn't break anything, but it's probably a bit
slower and not consistent with the others).
Merge pull request #881 from FioraAeterna/mulhwx

JIT64: add mulhwx implementation
Merge pull request #898 from FioraAeterna/fprffix

JIT: make fprf conditional in fcmp, just like the other instructions
Merge pull request #876 from FioraAeterna/floatloadstore

JIT64: clean up and unify float load/store code