[jdk20u] RFR: 8296412: Special case infinite loops with unmerged backedges in IdealLoopTree::check_safepts

[Tobias Hartmann]

Backport of [JDK-8296412](https://bugs.openjdk.java.net/browse/JDK-8296412). Applies cleanly. Approval is pending.

Thanks,
Tobias

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Commit messages:
 - 8296412: Special case infinite loops with unmerged backedges in IdealLoopTree::check_safepts

Changes: https://git.openjdk.org/jdk20u/pull/49/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk20u&pr=49&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296412
  Stats: 252 lines in 3 files changed: 252 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk20u/pull/49.diff
  Fetch: git fetch https://git.openjdk.org/jdk20u.git pull/49/head:pull/49

PR: https://git.openjdk.org/jdk20u/pull/49