[jdk21u-dev] RFR: 8334421: assert(!oldbox->is_unbalanced()) failed: this should not be called for unbalanced region

[Aleksey Shipilev]

This is a fix for regression introduced by [JDK-8324969](https://bugs.openjdk.org/browse/JDK-8324969), and already backported to `21.0.5`. This forces our hand for backporting it to 21u-dev.

Additional testing:
 - [x] New regression tests passes with the fix, and STILL PASSES without the fix
 - [ ] Linux x86_64 server fastdebug, `all`
 - [ ] Linux x86_64 server fastdebug, Maven CTW
 - [ ] Linux x86_64 server fastdebug, Fuzzer 100K

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Commit messages:
 - Backport 9c89f0861c1b6d25e1a7c3ac1add9a168d807788

Changes: https://git.openjdk.org/jdk21u-dev/pull/795/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=795&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334421
  Stats: 237 lines in 6 files changed: 231 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/795.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/795/head:pull/795

PR: https://git.openjdk.org/jdk21u-dev/pull/795