RFR: 8336098: G1: Refactor G1RebuildRSAndScrubTask

[Albert Mingkun Yang]

On Wed, 10 Jul 2024 13:26:31 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

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

I have reverted the `oop_oop_iterate` part, which sometime is perf-neutral or -8% regression from run to run...

Also modified `scan_and_scrub_region` a bit to make the enclosing two steps more symmetric and the whole method smaller.

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PR Comment: https://git.openjdk.org/jdk/pull/20114#issuecomment-2243677345