RFR: Improve mixed collection logging [v2]

[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.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Include old evac reserve in log message

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/281/files
  - new: https://git.openjdk.org/shenandoah/pull/281/files/f794060a..0a1f172a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=281&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=281&range=00-01

  Stats: 6 lines in 1 file changed: 2 ins; 0 del; 4 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