[jdk21u-dev] RFR: 8325074: ZGC fails assert(index == 0 || is_power_of_2(index)) failed: Incorrect load shift: 11

[Aleksey Shipilev]

Clean backport to fix a regression introduced by [JDK-8321619](https://bugs.openjdk.org/browse/JDK-8321619) in 21.0.3.

Additional testing:
 - [ ] Linux x86_64 server fastdebug, `all` with `-XX:+UseZGC -XX:+ZGenerational`

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Commit messages:
 - Backport 2705ed0a71e606a517518569d60051c85ad3c516

Changes: https://git.openjdk.org/jdk21u-dev/pull/268/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=268&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325074
  Stats: 5 lines in 3 files changed: 1 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/268.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/268/head:pull/268

PR: https://git.openjdk.org/jdk21u-dev/pull/268