RFR: 8342927: GenShen: Guarantee slices of time for coalesce and filling

[William Kemper]

Clean backport.

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Commit messages:
 - Backport 0eceb3574e17fd03fd739ce944eefb5c535d036c

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/132/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=132&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342927
  Stats: 14 lines in 2 files changed: 3 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/132.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/132/head:pull/132

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/132