RFR: 8326781: G1ConcurrentMark::top_at_rebuild_start() should take a HeapRegion* not an uint

[Thomas Schatzl]

Hi all,

  please review this refactoring to have `G1ConcurrentMark::top_at_rebuild_start` take a `HeapRegion*` instead of `uint` since all callers calculate the `uint` by calling `HeapRegion::hrm_index()` themselves in the argument.

Testing: gha, compilation

Thanks,
  Thomas

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Commit messages:
 - 8326781

Changes: https://git.openjdk.org/jdk/pull/18024/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18024&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326781
  Stats: 8 lines in 3 files changed: 0 ins; 1 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/18024.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18024/head:pull/18024

PR: https://git.openjdk.org/jdk/pull/18024