RFR: 8340490: Shenandoah: Optimize ShenandoahPacer

[William Kemper]

Not clean, one simple conflict.

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Commit messages:
 - Backport 65200a9589e46956a2194b20c4c90d003351a539

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/110/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=110&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340490
  Stats: 41 lines in 3 files changed: 8 ins; 16 del; 17 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/110.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/110/head:pull/110

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/110