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

[Kelvin Nilsen]

On Wed, 20 Apr 2022 16:10:49 GMT, William Kemper <wkemper at openjdk.org> wrote:

> 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.

Marked as reviewed by kdnilsen (Committer).

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