RFR: 8339611: GenShen: Simplify ShenandoahOldHeuristics::trigger_collection_if_fragmented

[Kelvin Nilsen]

This pull request contains a backport of commit c38b31af from the openjdk/shenandoah repository.

The commit being backported was authored by Kelvin Nilsen on 13 Sep 2024 and was reviewed by William Kemper.

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Commit messages:
 - Backport c38b31afe8ff4fdcdd27a771e72d38b128273f01

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/89/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=89&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339611
  Stats: 72 lines in 3 files changed: 25 ins; 15 del; 32 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/89.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/89/head:pull/89

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/89