This method is used to locate the next source-region to continue filling up the current destination-region (`PSParallelCompact::fill_region`). It has two exits, one for finding a non-empty region in the same source space, while the other for in another space.

The first arg of this method is used as an out-parameter, essentially `closure._source`, which should be corresponding to the returned region-index. One can just use the found region-index to calculate the intended value for `closure._source`.

Therefore, the condition check `if (src_region_addr > closure.source()) {` is not needed and often `true`, and `mark_bitmap()->find_obj_beg` is already handled by the caller. Remove both to streamline the flow.