RFR: 8330694: Rename 'HeapRegion' to 'G1HeapRegion' [v3]

[Lei Zaakjyu]

> follow up 8267941

Lei Zaakjyu has updated the pull request incrementally with one additional commit since the last revision:

  also tidy up

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/18871/files
  - new: https://git.openjdk.org/jdk/pull/18871/files/cae3efd0..f02334fd

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18871&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18871&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18871.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18871/head:pull/18871

PR: https://git.openjdk.org/jdk/pull/18871