JIT: fix a corner case in crXXX

It didn't behave correctly with an input of zero, resulting in some games
breaking (at the least, Fight Night 2). This should be fixed now.

Also clean it up, add a few comments, and fix some variants of the instruction
that are so rare that they probably never got tested.
Merge pull request #1035 from FioraAeterna/fixcrxxx

JIT: fix a corner case in crXXX