RFR: 8335395: G1: Verification does not detect references into Free regions

[Thomas Schatzl]

Hi all,

  please review this improvement to `G1CollectedHeap::is_obj_dead()` where references into free regions were inadvertedly considered as live. This makes verification not fail with a proper error message when encountering references to objects in free regions (it may still fail for other reasons).

The alternative would have been fixing this in verification code only, but that method does not seem that hot, and the additional code not too slow either, but feel free to tell me otherwise and have me move the check.

This has been verified by @christianhaeubl who reported the issue.

Testing: gha

Thanks,
  Thomas

-------------

Commit messages:
 - 8335395

Changes: https://git.openjdk.org/jdk/pull/19983/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19983&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335395
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/19983.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19983/head:pull/19983

PR: https://git.openjdk.org/jdk/pull/19983