[SCEVExpander] Only check overflow if it is needed.

[SCEVExpander] Only check overflow if it is needed.

9345ab3a4550 updated generateOverflowCheck to skip creating checks that
always evaluate to false. This in turn means that we only need to check
for overflows if the result of the multiplication is actually used.

Sink the Or for the overflow check into ComputeEndCheck, so it is only
created when there's an actual check.