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.
Merge pull request #876 from FioraAeterna/floatloadstore

JIT64: clean up and unify float load/store code