[jdk21u] RFR: 8315082: [REDO] Generational ZGC: Tests crash with assert(index == 0 || is_power_of_2(index))

[Aleksey Shipilev]

Clean backport to improve GC reliability.

Additional testing:
 - [ ] New regression tests fail without the patch, pass with it
 - [ ] Linux x86_64 fastdebug, `tier1 tier2 tier3 tier4`

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Commit messages:
 - Backport ceff47b462ccbaff5cc16111dc65463a6d8d3d8d

Changes: https://git.openjdk.org/jdk21u/pull/344/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=344&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315082
  Stats: 188 lines in 6 files changed: 157 ins; 10 del; 21 mod
  Patch: https://git.openjdk.org/jdk21u/pull/344.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/344/head:pull/344

PR: https://git.openjdk.org/jdk21u/pull/344