cmpi shall compare two signed 32 bit values. The used difference a-b
may overflow and so the resulting 32 bit value can't represent it.
A correct way would be cr = s64(a) - s64(b) and it should be done in
this way in the JITs, but the Interpreter shall implement the most
readable way.
Also drops the now unused helper function.
Markus Wick
b89e4b5258