RFR: Fix off-by-one error when handling pinned regions in a mixed collection [v2]

[William Kemper]

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

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

  Remove over aggressive asserts, improve readability

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/226/files
  - new: https://git.openjdk.org/shenandoah/pull/226/files/f6dad77b..cf7040fd

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=226&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=226&range=00-01

  Stats: 28 lines in 2 files changed: 1 ins; 22 del; 5 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