RFR: 8312422: GenShen: In-place region promotion state may carry over when evacuation fails

[William Kemper]

When a full gc starts, clear any state associated with a region scheduled for in-place promotion.

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Commit messages:
 - Restore top before promote when beginning a full GC

Changes: https://git.openjdk.org/shenandoah/pull/299/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=299&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312422
  Stats: 9 lines in 1 file changed: 9 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/299.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/299/head:pull/299

PR: https://git.openjdk.org/shenandoah/pull/299