RFR: 8311599 GenShen: Missing card mark barrier when processing references

[William Kemper]

When reference processing threads operate on the discovered list, they may create old-to-young pointers. Such pointers need to be recorded in the remembered set.

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Commit messages:
 - Use card mark barrier when updating discovered list

Changes: https://git.openjdk.org/shenandoah/pull/294/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=294&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311599
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/294.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/294/head:pull/294

PR: https://git.openjdk.org/shenandoah/pull/294