RFR: 8328932: Parallel: Proper partial object setup in fill_dense_prefix_end [v2]

[Albert Mingkun Yang]

> Oneline diff  to maintain the partial-obj invariant, just like `MarkingStatsCache::push`.

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 one additional commit since the last revision:

  pgc-filler-partial-obj

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18469/files
  - new: https://git.openjdk.org/jdk/pull/18469/files/ceb99058..0bf7cabd

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18469&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18469&range=00-01

  Stats: 10766 lines in 261 files changed: 4482 ins; 4550 del; 1734 mod
  Patch: https://git.openjdk.org/jdk/pull/18469.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18469/head:pull/18469

PR: https://git.openjdk.org/jdk/pull/18469