RFR: 8319671 GenShen: Old marking may encounter invalid pointers in SATB buffers

[William Kemper]

Weak handles which are null'd out after final marking of young, but during marking of old, will have their original referent placed into a SATB buffer. These entries need to be purged before old marking resumes. In the case of abbreviated young cycles, these entries would not be purged.

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Commit messages:
 - Purge SATB buffers if necessary at the end of an abbreviated cycle

Changes: https://git.openjdk.org/shenandoah/pull/353/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=353&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319671
  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/353.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/353/head:pull/353

PR: https://git.openjdk.org/shenandoah/pull/353