Optimize sw/lw even under safe memory.

They're very common instructions, so shaving cycles helps.
Also jit sh and lhu, which are pretty common ops.
Avoid a func if possible when fastmem is off.
Gotta flush before the call, too.
Merge pull request #440 from unknownbrackets/jit-minor

Micro optimizations to x86 jit mem when fastmem is off