RFR: Use state from collection set to know if mixed collection is in progress

[William Kemper]

The state that indicated when a mixed collection was in progress was represented in two places: a property of the collection set and a member variable on the heap set after the collection set was chosen. In some cases, a degenerated cycle could cause the heap's member variable to become stale (indicating a mixed evac was in progress when it was not). In this case, update references could fail to update references in old regions with an incomplete mark bitmap. With this change, update references uses the property of the collection set to know if a mixed evacuation is in progress.

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Commit messages:
 - Use state from collection set to know if mixed collection is in progress

Changes: https://git.openjdk.java.net/shenandoah/pull/134/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=134&range=00
  Stats: 42 lines in 11 files changed: 0 ins; 23 del; 19 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/134.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/134/head:pull/134

PR: https://git.openjdk.java.net/shenandoah/pull/134