RFR: 8331385: G1: Prefix HeapRegion helper classes with G1

[Thomas Schatzl]

Hi all,

  after [JDK-8330694](https://bugs.openjdk.org/browse/JDK-8330694) which renamed HeapRegion to G1HeapRegion, there were  a few related helper classes in this CR that were not renamed.

It's purely mechanical renaming without even further renaming of files etc.

This change updates them.

(Fwiw, the "Viewed" checkbox at the top right of the file change helps a lot review this change incrementally)

Testing: tier1, tier4, tier5

Thanks,
  Thomas

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Commit messages:
 - 8331385

Changes: https://git.openjdk.org/jdk/pull/19967/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19967&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331385
  Stats: 887 lines in 68 files changed: 163 ins; 165 del; 559 mod
  Patch: https://git.openjdk.org/jdk/pull/19967.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19967/head:pull/19967

PR: https://git.openjdk.org/jdk/pull/19967