JIT: implement frsqrte

Mostly a straightforward translation of the interpreter code, with a few
tricksy optimizations and fallbacks for rare paths.
JIT: implement fres

Mostly a straightforward translation of the interpreter code, with a few
tricksy optimizations and fallbacks for rare paths.
Merge pull request #947 from FioraAeterna/rsqrte

JIT: implement frsqte