Currently bool PhaseIdealLoop::is_counted_loop(), despite what the name suggests, also converts the loop, making calling it non-idempotent.

As discussed during the review of JDK-8336759 [0], this should be refactored into at least two functions. This issue is created as a dependency of JDK-8336759 due to its complexity and should be reviewed separately.

[0] https://github.com/openjdk/jdk/pull/22449#issuecomment-2520215899