Jit for sb, further optimizations

Well, I couldn't reproduce the problems with sb anymore (maybe because I'm flushing less?) on 32-bit, but what you said before makes sense.  So I made it use EDX if it wasn't already in EDX/ECX.

Then I optimized the code a bit, to avoid using EAX as a temporary if possible.  This means that:
 * On x64 w/fastmem, most reads/writes will be a single instruction.
 * On x86/x64 w/slowmem, most reads/writes will just be that plus 2 CMPs and a JMP.

-[Unknown]