RFR: Fix off-by-one error when handling pinned regions in a mixed collection

[William Kemper]

This fixes an off-by-one error that manifested when the first mixed collection candidate was pinned.

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Commit messages:
 - Stronger unit test assertions
 - Update comments and assertion checks
 - Fix off-by-one error when all regions are pinned
 - Remove unfinished unit test
 - Merge branch 'shenandoah-master' into fix-old-pinned
 - WIP: Fix off by one error, improve unit test

Changes: https://git.openjdk.org/shenandoah/pull/226/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=226&range=00
  Stats: 97 lines in 2 files changed: 45 ins; 4 del; 48 mod
  Patch: https://git.openjdk.org/shenandoah/pull/226.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/226/head:pull/226

PR: https://git.openjdk.org/shenandoah/pull/226