RFR: Account for usage of all regions in humongous object during promotion

[earthling-amzn]

Fixes bug in how usage is accounted for in generations.

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Commit messages:
 - Account for usage of all regions in humongous object during promotion

Changes: https://git.openjdk.java.net/shenandoah/pull/20/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=20&range=00
  Stats: 5 lines in 1 file changed: 2 ins; 3 del; 0 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/20.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/20/head:pull/20

PR: https://git.openjdk.java.net/shenandoah/pull/20