RFR: More promotion fixes

[William Kemper]

This PR addresses a few issues with region promotion:
* Fix typo in code which loops over continuation regions of humongous objects
* Don't rely on mark bits above TAMS when iterating oops
* Due to an as-yet-undiagnosed bug, we need to mark the cards for an entire region dirty when it's promoted

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Commit messages:
 - Use card marking abstraction in heap
 - Revert unnecessary change
 - Use cast_from_oop rather than C-style cast.
 - Dirty entire regions during promotion.
 - Operate on region in loop, rather than 'this' region
 - Don't rely on mark bit map above TAMS when iterating oops in region.

Changes: https://git.openjdk.java.net/shenandoah/pull/39/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=39&range=00
  Stats: 31 lines in 4 files changed: 16 ins; 2 del; 13 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/39.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/39/head:pull/39

PR: https://git.openjdk.java.net/shenandoah/pull/39