RFR: Improve mixed collection logging

[Y. Srinivas Ramakrishna]

On Thu, 18 May 2023 23:16:55 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Not including any old regions in a mixed collection should be rare. It could indicate a bug. This change will log the relevant state when this happens.

Marked as reviewed by ysr (Committer).

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PR Review: https://git.openjdk.org/shenandoah/pull/281#pullrequestreview-1433589341