RFR: 8301222: Generalize check_release_entry in OopStorage

[Erik Österlund]

When an oop handle is released to OopStorage, we want to check that its contents has been cleared appropriately, honouring the general contract with OopStorage. This is currently done with a raw access checking for null. However, the raw contents in memory might not be 0, just because the logical value is null. In particular, generational ZGC will have some low order colour bits set. This CR aims to address that.

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Commit messages:
 - Trying the contains_null approach
 - Merge branch 'master' into 8301222_check_release_entry
 - 8301222: Generalize check_release_entry in OopStorage

Changes: https://git.openjdk.org/jdk/pull/12252/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12252&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301222
  Stats: 11 lines in 3 files changed: 10 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12252.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12252/head:pull/12252

PR: https://git.openjdk.org/jdk/pull/12252