RFR: 8337981: ShenandoahHeap::is_in should check for alive regions

[William Kemper]

Need this to complete partial backport of related work in early change.

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Commit messages:
 - 8337981: ShenandoahHeap::is_in should check for alive regions

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/181/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=181&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337981
  Stats: 103 lines in 10 files changed: 49 ins; 7 del; 47 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/181.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/181/head:pull/181

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/181