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
Shouldn't we be handling this case in SafeWriteRegToReg?  It seems much more straightforward to make it do the right thing.  (Granted, UnsafeWriteRegToReg completely screws this up at the moment, but that shouldn't be hard to fix; see UnsafeLoadToReg.)