RFR: 8341099: GenShen: assert(HAS_FWD == _heap->has_forwarded_objects()) failed: Forwarded object status is sane

[William Kemper]

Evacuations may occur during young or mixed collection cycles. When it is a young cycle during old marking, this barrier may be called to both evacuate cset objects and again to enqueue the targets of the copy (if they are in the old generation). If it is a mixed collection, then old marking will _not_ be in progress and this barrier will not be called to enqueue anything.

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Commit messages:
 - Relax arraycopy assertion to allow for evacuations during old generation marking call

Changes: https://git.openjdk.org/shenandoah/pull/507/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=507&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341099
  Stats: 10 lines in 1 file changed: 8 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/507.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/507/head:pull/507

PR: https://git.openjdk.org/shenandoah/pull/507