[SCEV] Add function to compute minium of countable exits.

This patch introduces getBackedgeTakenCountForCountableExits and a
predicated version to compute the minimum of the countable exits.

The intended use for this is loop access analysis for loops with
uncountable exits. When analyzing dependences and computing runtime
checks, we need the smallest upper bound on the number of iterations. In
terms of memory safety, it shouldn't matter if any uncomputable exits
leave the loop, as long as we prove that there are no dependences given
the minimum of the countable exits. The same should apply also for
generating runtime checks.