x86jit/ppcjit: Correct some bad sltiu compares

The one in x86jit is a bit nasty in the cases where it might get hit.  Well, the ppc one is even more dangerous but not really active atm.

In the vast majority of cases, theoretically, it would be a loop counting from 0, and so it'd be fine.  But there has to be somewhere it's a nasty bug (and I'm always saying how surprised I am that these sort of known-result instructions occur even outside of loops...)

-[Unknown]