Parallel: Refactor PSParallelCompact::fill_dense_prefix_end

The purpose of this method is to create a filler object just before the prefix-end to enable parsing of the [bottom, prefix-end] range. The current implementation/documentation outlines all possible scenarios and the corresponding actions.

However, there's only one scenario where a filler object is required: when there is a single heap word gap (dead object) right before the prefix-end.

Simplify the implementation/documentation to highlight this single scenario.