More x86 jit stuff

Adds a lot more immediate handling, mostly because I wanted to confirm some suspicions: sometimes branches are done with values known at compile time (even rs/rt.)  At first I was surprised but then I realized this must be for/while loops.

Anyway, probably doesn't improve performance a ton, although the immediate branches I added were all hit iirc.

Not really sure the best way to implement clz/clo... tried BSR with a JNZ and 31 - EAX as the result, but must've been doing something wrong since it didn't work... not that it's super important to jit, anyway.

-[Unknown]