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

[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

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Commit messages:
 - * remove accidentally merged in JDK-8364934
 - 8364925

Changes: https://git.openjdk.org/jdk/pull/26659/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26659&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364925
  Stats: 51 lines in 2 files changed: 27 ins; 16 del; 8 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