In `PSParallelCompact::verify_filler_in_dense_prefix`, we verify all dead space is filled with special fillers, with their corresponding klass. The ordinary klass accessor API is `klass()`. There is another variant, `klass_without_asserts`, which is used in contexts where it's dangerous to access the klass.
In this context, klass must be one of special filler klasses, so one can use the ordinary version.

Change to the ordinary version to streamline the logic in this method.