RFR: 8336098: G1: Refactor G1RebuildRSAndScrubTask

[Albert Mingkun Yang]

Small refactoring to consolidate the early-return logic during scan-and-rebuild.

Test: tie1-3

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Commit messages:
 - g1-conc-rebuild

Changes: https://git.openjdk.org/jdk/pull/20114/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20114&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336098
  Stats: 119 lines in 1 file changed: 22 ins; 74 del; 23 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