RFR: 8352969: G1: Improve testability of optional collections [v2]

[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 incrementally with one additional commit since the last revision:

  Simplified implementation

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/26880/files
  - new: https://git.openjdk.org/jdk/pull/26880/files/22420f09..6661deef

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26880&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26880&range=00-01

  Stats: 18 lines in 3 files changed: 5 ins; 0 del; 13 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