RFR: Include old humongous start regions when counting immediate garbage

[William Kemper]

We only log these values, but we expect to one day use them to possibly shortcut an old generation collection in the same fashion as the young generation.

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Commit messages:
 - Include old humongous start regions when counting immediate garbage

Changes: https://git.openjdk.org/shenandoah/pull/157/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=157&range=00
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/157.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/157/head:pull/157

PR: https://git.openjdk.org/shenandoah/pull/157