G1: Refactor G1CardSet::get_card_set to return G1CardSetHashTableValue*

G1CardSet::get_card_set should return G1CardSetHashTableValue* similar to G1CardSet::get_or_add_card_set. In this way, we don't have to use G1CardSet::get_or_add_card_set in instances where we only need to get the table entry.