[jdk16u] RFR: 8265239: Shenandoah: Shenandoah heap region count could be off by 1

[Zhengyu Gu]

8265239: Shenandoah: Shenandoah heap region count could be off by 1

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Commit messages:
 - Backport 5a526c1c5716f6d9a7fc94741bcdb2f424d342df

Changes: https://git.openjdk.java.net/jdk16u/pull/110/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16u&pr=110&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265239
  Stats: 2 lines in 2 files changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk16u/pull/110.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16u pull/110/head:pull/110

PR: https://git.openjdk.java.net/jdk16u/pull/110