RFR: 8343508: Parallel: Use ordinary klass accessor in verify_filler_in_dense_prefix

[Albert Mingkun Yang]

One line change to use the common API to make the caller logic less obtrusive.

Test: tier1-3

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Commit messages:
 - pgc-klass-accessor

Changes: https://git.openjdk.org/jdk/pull/21866/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21866&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343508
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21866.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21866/head:pull/21866

PR: https://git.openjdk.org/jdk/pull/21866