[LAA] Delay applying loop guards until after isSafeDependenceDistance.

Applying the loop guards to the distance may prevent
isSafeDependenceDistance from determining NoDep, unless loop guards are
also applied to the backedge-taken-count.

Instead of applying the guards to both Dist and the
backedge-taken-count, just apply them after handling
isSafeDependenceDistance and constant distances; there is no benefit to
applying the guards before then.

This fixes a regression flagged by @bjope due to
ecae3ed9.