[LoopInterchange] Adjust assertions when updating successors.

Currently the assertion in updateSuccessor is overly strict in some
cases and overly relaxed in other cases. For branches to the inner and
outer loop preheader it is too strict, because they can either be
unconditional branches or conditional branches with duplicate targets.
Both cases are fine and we can allow updating multiple successors.

On the other hand, we have to at least update one successor. This patch
adds such an assertion.