RFR: 8325725: Parallel: Refactor PSParallelCompact::fill_dense_prefix_end

[Albert Mingkun Yang]

Refactor the logic around creating filler obj before prefix-end.

Test: tier1-6

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Commit messages:
 - pgc-end-bit

Changes: https://git.openjdk.org/jdk/pull/17822/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17822&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325725
  Stats: 103 lines in 3 files changed: 20 ins; 67 del; 16 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