RFR: 8312321: GenShen: Remembered set scan may encounter garbage objects

[William Kemper]

During a global cycle, regions with no live objects may be reclaimed early (before evacuation). Once this happens, we _must_ coalesce and fill any dead objects in old regions because they may hold pointers into the reclaimed regions (even if the gc is cancelled before coalesce and fill). A subsequent scan of the remembered set may crash if it encounters these objects.

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Commit messages:
 - Coalesce objects for concurrent global GC even if the GC is cancelled

Changes: https://git.openjdk.org/shenandoah/pull/297/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=297&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312321
  Stats: 8 lines in 1 file changed: 4 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/shenandoah/pull/297.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/297/head:pull/297

PR: https://git.openjdk.org/shenandoah/pull/297