RFR: 8328792: Parallel: Refactor PSParallelCompact::summary_phase [v6]

[Albert Mingkun Yang]

> The diff is not easy to follow; it's better to read the new code directly. `summary_phase` contains the gist of collecting live-size from each space, calculating dense-prefix and summarizing old-space using dense-prefix.
> 
> (There is slight improvement to the "Summary Phase", but it takes only ~0.01ms for either case.)

Albert Mingkun Yang has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains one commit:

  pgc-compute-dense-prefix

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Changes: https://git.openjdk.org/jdk/pull/18451/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18451&range=05
  Stats: 267 lines in 2 files changed: 91 ins; 148 del; 28 mod
  Patch: https://git.openjdk.org/jdk/pull/18451.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18451/head:pull/18451

PR: https://git.openjdk.org/jdk/pull/18451