[IndVars] Remove monotonic checks with unknown exit count

[IndVars] Remove monotonic checks with unknown exit count

Even if the exact exit count is unknown, we can still prove that this
exit will not be taken. If we can prove that the predicate is monotonic,
fulfilled on first & last iteration, and no overflow happened in between,
then the check can be removed.

Differential Revision: https://reviews.llvm.org/D87832
Reviewed By: apilipenko