RFR: Improve mixed collection logging

[William Kemper]

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.

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Commit messages:
 - Fix typo
 - Merge remote-tracking branch 'shenandoah/master' into improve-mixed-collection-logging
 - Log when and why no regions are selected for mixed collection

Changes: https://git.openjdk.org/shenandoah/pull/281/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=281&range=00
  Stats: 13 lines in 1 file changed: 12 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/281.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/281/head:pull/281

PR: https://git.openjdk.org/shenandoah/pull/281