[SCEV] Generalize no-self-wrap check in isLoopInvariantExitCondDuringFirstIterations

Lift limitation on step being `+/- 1`. In fact, the only thing it is needed for
is proving no-self-wrap. We can instead check this flag directly.

Theoretically it can increase the scope of the transform, but I could not
construct such test easily.

Differential Revision: https://reviews.llvm.org/D91126
Reviewed By: apilipenko