RFR: 8304393: Provide method to iterate over regions of humongous object in G1

[Thomas Schatzl]

Hi all,

  please review this refactoring that introduces a method to iterate over all regions of a humongous object and apply a function to it to decrease the five times code duplication.

Testing: gha

Thanks,
  Thomas

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Commit messages:
 - Fix indentation
 - Initial implementation
 - initial non-working version

Changes: https://git.openjdk.org/jdk/pull/13080/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13080&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304393
  Stats: 73 lines in 7 files changed: 26 ins; 23 del; 24 mod
  Patch: https://git.openjdk.org/jdk/pull/13080.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/13080/head:pull/13080

PR: https://git.openjdk.org/jdk/pull/13080