In JDK-8348572, we simply decided to bail out when we discover a new irreducible loop. But we should prevent even creating them.

Reiterating from JDK-8348572:
We would like JDK-8348570 to be unblocked - that is why JDK-8348572 just bails out silently when it sees a new irreducible loop that was not already tracked by parsing.

What we should do now:
Investigate what we can do to avoid creating new irreducible loops in split_if. Detailed explanations can be found in JDK-8348572 / https://git.openjdk.org/jdk/pull/23363.

For now, have the assert behind a Verify flag, so that JDK-8348570 is unblocked. We should remove the Verify flag and alway enable the assert again.