JIT: some more integer optimizations

This isn't that heavily tested so it could probably use a JMC pass or two and a sanity check by some other eyes
Well, it worked because you were lucky and it was already in a reg, heh.

I've actually thought about flushing a flag on each reg per instruction.  So, if you call RX on it and the flag is not set, it throws - even if it really is in a reg.  The flag would have to be set by IsSimpleReg and MapReg/BindToRegister.  Have not thought it through fully though...

-[Unknown]