RFR: 8345293: Fix generational Shenandoah with compact headers

[Roman Kennke]

See bug for crash details.

The problem is in the code that gets the object age out of the mark-word. That code has a special cases for when an object is monitor locked, in which case it fetches the displaced header out of the monitor and extracts the age from there. However, with compact headers, we're running with ObjectMonitorTable, and decoding the monitor-locked mark-word crashes.

The fix is simple: when we are running with ObjectMonitorTable, the mark-word never gets overloaded by locking, so we can return the age straight out of the mark-word.

Testing:
 - [x] hotspot_gc_shenandoah +UCOH

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Commit messages:
 - 8345293: Fix generational Shenandoah with compact headers

Changes: https://git.openjdk.org/jdk/pull/22477/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22477&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345293
  Stats: 5 lines in 1 file changed: 5 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/22477.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22477/head:pull/22477

PR: https://git.openjdk.org/jdk/pull/22477