Jit known prefix support

This makes known prefixes work.  I mucked around a lot in the FPR cache, hope the changes are okay.

It does what's simple:
 * Adds 16 temp regs just in case (need 4 for each of S and T, 4 to make D masks easy, and +4 in case an instruction needs them.)
 * Since there's more than 6, adds a way to store the regs temporarily if they're swapped out.  Probably could be optimized to not swap them out if possible better.
 * Uses temp regs for S/T modified values.
 * Uses temp regs for D masked values (seemed like the easiest way.)

Tested a bunch of permutations of prefixes/regs and seems to work great (at least as well as the interpreter.)

-[Unknown]