[LV] Handle zero cost loops in selectInterleaveCount.

In some case, like in the added test case, we can reach
selectInterleaveCount with loops that actually have a cost of 0.

Unfortunately a loop cost of 0 is also used to communicate that the cost
has not been computed yet. To resolve the crash, bail out if the cost
remains zero after computing it.

This seems like the best option, as there are multiple code paths that
return a cost of 0 to force a computation in selectInterleaveCount.
Computing the cost at multiple places up front there would unnecessarily
complicate the logic.

Fixes #54413.