RFR: 8336098: G1: Refactor G1RebuildRSAndScrubTask [v3]

[Albert Mingkun Yang]

> Small refactoring to consolidate the early-return logic during scan-and-rebuild.
> 
> Test: tie1-3

Albert Mingkun Yang has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:

 - review
 - Merge branch 'master' into g1-conc-rebuild
 - Merge branch 'master' into g1-conc-rebuild
 - review
 - g1-conc-rebuild

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20114/files
  - new: https://git.openjdk.org/jdk/pull/20114/files/a6ecec0f..36f6f355

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20114&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20114&range=01-02

  Stats: 1143 lines in 62 files changed: 786 ins; 178 del; 179 mod
  Patch: https://git.openjdk.org/jdk/pull/20114.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20114/head:pull/20114

PR: https://git.openjdk.org/jdk/pull/20114