RFR: 8328602: Parallel: Incorrect assertion in fill_dense_prefix_end

[Albert Mingkun Yang]

Relax too-strong assertion about min-filler-size to account for large obj-alignment.

Test: with explicit large obj-alignment

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Commit messages:
 - pgc-obj-alignment

Changes: https://git.openjdk.org/jdk/pull/18396/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18396&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328602
  Stats: 20 lines in 1 file changed: 13 ins; 1 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/18396.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18396/head:pull/18396

PR: https://git.openjdk.org/jdk/pull/18396