RFR: 8364925: G1: Improve program flow around incremental collection set building [v2]

[Thomas Schatzl]

> Hi all,
> 
>   please review this change that improves the G1CollectionSet incremental state handling slightly:
> 
> * remove `_selected_groups_cur_length` with getter as we do not need a copy of that value around
> * move implementations of some methods from header files into cpp file
> * moved some incremental state updates from `finalize_old_part` to the respective `start/continue/stop_incremental_building` method
> 
> Testing: gha
> 
> Thanks,
>   Thomas

Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:

  * ayang review

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/26659/files
  - new: https://git.openjdk.org/jdk/pull/26659/files/141d18df..1c8188fa

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26659&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26659&range=00-01

  Stats: 3 lines in 2 files changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/26659.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26659/head:pull/26659

PR: https://git.openjdk.org/jdk/pull/26659