[jdk20] RFR: 8300275: SegmentScope.isAccessibleBy returning incorrect values

[Maurizio Cimadamore]

The implementation of MemorySessionImpl::isAccessibleBy is incorrect, and ends up always returning `false` for scopes associated with shared arenas.
This patch rectifies that, and adds some tests.

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Commit messages:
 - Fix bug in MemorySessionImpl::isAccessibleBy

Changes: https://git.openjdk.org/jdk20/pull/110/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk20&pr=110&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300275
  Stats: 24 lines in 2 files changed: 19 ins; 2 del; 3 mod
  Patch: https://git.openjdk.org/jdk20/pull/110.diff
  Fetch: git fetch https://git.openjdk.org/jdk20 pull/110/head:pull/110

PR: https://git.openjdk.org/jdk20/pull/110