x86jit: More fpu optimizations

These gain 10-15 points in a few games I tested, which is only around 1%, but might be better on some architectures or games.

Specifically:
 * Implements fpu rounding conversions in jit other than cvt.w.s (used by some games, e.g. Kingdom Hearts.)
 * Fixes a few places where regs were stored to mem and then overwritten anyway, or otherwise unnecessarily.
 * Optimizes the case of loading a known immediate 0 into an fpu reg.
 * Small improvement to 3-ops of the form "add.s x, y, y" / "mul.s x, y, y".
 * Kills a couple minor MOVSS to self.  Maybe the emitter should nix that, even...

-[Unknown]
We use RBX as the memory base pointer in 64-bit, but it does appear that we could actually use it  in 32-bit, I can't find a reason why not...