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).
- neg can only overflow, if a == MIN
- case `-0` is properly handled by hardware, so overflow check by comparing
`a == MIN` is sufficient
- tests: MIN, MIN+1, MIN+4, -42, -7, -1, 0, 1, 7..
See #1290