RFR: 8325725: Parallel: Refactor PSParallelCompact::fill_dense_prefix_end [v3]

[Albert Mingkun Yang]

> Refactor the logic around creating filler obj before prefix-end.
> 
> Test: tier1-6

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-end-bit
 - pgc-end-bit

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/17822/files
  - new: https://git.openjdk.org/jdk/pull/17822/files/952823ee..a5ac13d8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17822&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17822&range=01-02

  Stats: 7902 lines in 1024 files changed: 3532 ins; 1514 del; 2856 mod
  Patch: https://git.openjdk.org/jdk/pull/17822.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17822/head:pull/17822

PR: https://git.openjdk.org/jdk/pull/17822