RFR: 8352969: G1: Improve testability of optional collections [v7]

[Guanqiang Han]

> This PR introduces a new diagnostic flag EvacuateAllOptionalRegions to force G1 GC to evacuate all optional regions regardless of the predicted pause time. The motivation is to allow testing and validation of optional region evacuation behavior without being constrained by the remaining pause time.

Guanqiang Han has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 10 commits:

 - change flag name
 - Merge remote-tracking branch 'upstream/master' into 8352969
 - fix format error
 - format fix
 - Optimize implementation and add regression test
 - Merge remote-tracking branch 'upstream/master' into 8352969
 - Update g1YoungCollector.cpp
   
   format fix
 - Simplified implementation
 - add default value
 - Implement JDK-8352969

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Changes: https://git.openjdk.org/jdk/pull/26880/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26880&range=06
  Stats: 126 lines in 4 files changed: 123 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/26880.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26880/head:pull/26880

PR: https://git.openjdk.org/jdk/pull/26880