tree-optimization/112344 - relax final value-replacement fix
The following tries to reduce the number of cases we use an unsigned
type for the addition when we know the original signed increment was
OK which is when the total unsigned increment computed fits the signed
type as well.
This fixes the observed testsuite fallout.
PR tree-optimization/112344
* tree-chrec.cc (chrec_apply): Only use an unsigned add
when the overall increment doesn't fit the signed type.