[jdk21u-dev] RFR: 8320888: Shenandoah: Enable ShenandoahVerifyOptoBarriers in debug builds

[Aleksey Shipilev]

Clean backport to improve Shenandoah maintainability.

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Commit messages:
 - Backport c86431767e6802317dc2be6221a5d0990b976ddc

Changes: https://git.openjdk.org/jdk21u-dev/pull/148/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=148&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320888
  Stats: 39 lines in 4 files changed: 0 ins; 37 del; 2 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/148.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/148/head:pull/148

PR: https://git.openjdk.org/jdk21u-dev/pull/148