RFR: 8312321: GenShen: Remembered set scan may encounter garbage objects [v2]

[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.

William Kemper has updated the pull request incrementally with two additional commits since the last revision:

 - Fix trailing whitespace
 - Fix typo in comment

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/297/files
  - new: https://git.openjdk.org/shenandoah/pull/297/files/b819a47c..2b959c8f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=297&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=297&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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