This improves readability as well as compatibility with stage2. Most of
compiler-rt is now enabled for stage2 with just a few functions disabled
(until stage2 passes more behavior tests).
- adds __cmpsi2, __cmpdi2, __cmpti2
- adds __ucmpsi2, __ucmpdi2, __ucmpti2
- use 2 if statements with 2 temporaries and a constant
- tests: MIN, MIN+1, MIN/2, -1, 0, 1, MAX/2, MAX-1, MAX if applicable
See #1290