RFR: Fix loop error when reconstructing remembered set for humongous objects

[William Kemper]

Also a small fix to avoid public `get_update_watermark` method when logging changes to a region affiliation.

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Commit messages:
 - Avoid public API for update watermark when logging
 - End of object will be top of region, not bottom

Changes: https://git.openjdk.java.net/shenandoah/pull/71/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=71&range=00
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/71.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/71/head:pull/71

PR: https://git.openjdk.java.net/shenandoah/pull/71