Use EDX as a temporary for sb, and jit it.
Avoid using EAX as a temporary where possible.

All the regs should be indirect addressing compatible.  So if it's
in a reg, let's use that instead of EAX.
Optimize write to a single x64 op too.
Same optimization for FPU load / store.
Merge pull request #456 from unknownbrackets/jit-slowmem

Jit for sb, further optimizations