RFR: 8368006: Parallel: Skip full regions in dense prefix during Full GC [v2]

[Albert Mingkun Yang]

> Implement skip-full-region optimization for filler related operations on dense-prefix. I constrained the filler-creation task to a single worker for simpler implementation. During the compaction phase, the most expensive part is almost always the non-dense-prefix part, so letting the rest of workers to start with the expensive tasks seems sensible.
> 
> Synthetic bms shows good reduction in full-gc time in debug build; perf-neutral for other bms (specjvm2008,specjbb2015) in release build.
> 
> Test: tier1-5

Albert Mingkun Yang has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - review
 - Merge branch 'master' into pgc-skip-full-region
 - pgc-skip-full-region

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27369/files
  - new: https://git.openjdk.org/jdk/pull/27369/files/3e73200f..20937445

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27369&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27369&range=00-01

  Stats: 6060 lines in 138 files changed: 4594 ins; 1188 del; 278 mod
  Patch: https://git.openjdk.org/jdk/pull/27369.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27369/head:pull/27369

PR: https://git.openjdk.org/jdk/pull/27369