[NFC][SCEV] Generalize monotonicity check for full and limited iteration space

A piece of logic of `isLoopInvariantExitCondDuringFirstIterations` is actually
a generalized predicate monotonicity check. This patch moves it into the
corresponding method and generalizes it a bit.

Differential Revision: https://reviews.llvm.org/D90395
Reviewed By: apilipenko