[jdk21u-dev] RFR: 8327423: C2 remove_main_post_loops: check if main-loop belongs to pre-loop, not just assert

[Martin Doerr]

Backport of [JDK-8327423](https://bugs.openjdk.org/browse/JDK-8327423). I only had to integrate loopTransform.cpp manually due to unrelated context differences.

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Commit messages:
 - Backport fadc4b197e927cfa1814fe6cb65ee04b3bd4b0c2

Changes: https://git.openjdk.org/jdk21u-dev/pull/701/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=701&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327423
  Stats: 64 lines in 2 files changed: 61 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/701.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/701/head:pull/701

PR: https://git.openjdk.org/jdk21u-dev/pull/701