RFR: 8342329: G1: Rename G1HeapRegionManager::_allocated_heapregions_length

[Ivan Walulya]

Hi,

Please review the refactor rename of G1HeapRegionManager::_allocated_heapregions_length to G1HeapRegionManager::_next_highest_used_hrm_index which better reflects the purpose of the field.

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Commit messages:
 - init

Changes: https://git.openjdk.org/jdk/pull/21530/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21530&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342329
  Stats: 21 lines in 2 files changed: 0 ins; 0 del; 21 mod
  Patch: https://git.openjdk.org/jdk/pull/21530.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21530/head:pull/21530

PR: https://git.openjdk.org/jdk/pull/21530