RFR: 8328932: Parallel: Proper partial object setup in fill_dense_prefix_end

[Albert Mingkun Yang]

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

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Commit messages:
 - pgc-filler-partial-obj

Changes: https://git.openjdk.org/jdk/pull/18469/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18469&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328932
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 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