x86jit: Micro optimize cvt.s.w a bit

This implementation is about 5x faster for micro benchmarks.  Little impact to overall perf in games I tested, though.

CVTDQ2PS has lower latency, and I suppose a game is likely to use the reg afterward, so might as well map fd.  Just a single op if fd == fs which I'm pretty sure is common, otherwise an unaligned load (maybe could map to a reg but unsure if losing a reg is worth it since they're likely done with that reg...)

-[Unknown]