[SCEV] Expose and use maximum constant exit counts for individual loop exits

We were already going to all of the trouble of computing maximum constant exit counts for each loop exit, we might as well expose them through the API.  The change in IndVars is mostly to demonstrate that the wired up code works, but it als very slightly strengthens the transform.  The strengthened case is rather narrow though: it requires one exactly analyzeable exit, one imprecisely analyzeable exit (with the upper bound less than the precise one), and one unanalyzeable exit.  I coudn't construct a reasonably stable test case.

This does increase the memory usage of the BackedgeTakenCount by a factor of 2 in the worst case.

I also noticed the loop in IndVars is O(#Exits ^ 2).  This doesn't change with this patch.  A future patch will cache this result inside of SCEV to avoid requering.

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