[jdk17u-dev] RFR: 8296412: Special case infinite loops with unmerged backedges in IdealLoopTree::check_safepts

[Goetz Lindenmaier]

I backport this for parity with 17.0.8-oracle.

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Commit messages:
 - Backport da38d43fcc640ea9852db6c7c23817dcef7080d5

Changes: https://git.openjdk.org/jdk17u-dev/pull/1287/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=1287&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/jdk17u-dev/pull/1287.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/1287/head:pull/1287

PR: https://git.openjdk.org/jdk17u-dev/pull/1287