[DA] control compile-time spent by MIV tests

Function exploreDirections() in DependenceAnalysis implements a recursive
algorithm for refining direction vectors. This algorithm has worst-case
complexity of O(3^(n+1)) where n is the number of common loop levels.
In this patch I'm adding a threshold to control the amount of time we
spend in doing MIV tests (which most of the time end up resulting in over
pessimistic direction vectors anyway).

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D107159