RFR: 8308088: Improve class check in CollectedHeap::is_oop

[Aleksey Shipilev]

See the bug for more details.

Additional testing:
 - [x] Linux x86_64 fastdebug, `tier1`
 - [x] Linux x86_64 fastdebug, `tier2`
 - [x] Linux x86_64 fastdebug, `tier3`

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/13982/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13982&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308088
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13982.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13982/head:pull/13982

PR: https://git.openjdk.org/jdk/pull/13982