RFR: 8274178: G1: Occupancy value in IHOP logging and JFR event is inaccurate

[Thomas Schatzl]

Hi all,

  please review this change that modifies heap occupancy reporting for IHOP to use the actual occupancy value the IHOP calculation actually uses.

I.e. previously IHOP logging used to report `G1CollectedHeap::used()` as occupancy (which is correct at a general level), but for IHOP/marking that value is not interesting, but the "occupancy" value used for comparing against the current heap occupancy threshold.

Testing: tier1-3, GHA

Thomas

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Commit messages:
 - 8274178

Changes: https://git.openjdk.org/jdk/pull/28175/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28175&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8274178
  Stats: 95 lines in 7 files changed: 42 ins; 30 del; 23 mod
  Patch: https://git.openjdk.org/jdk/pull/28175.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28175/head:pull/28175

PR: https://git.openjdk.org/jdk/pull/28175