cost_sort() improvements

failed: regress/enum, regress/rangetypes, regress/pg_lsn, regress/numerology, regress/type_sanity, regress/opr_sanity, regress/insert, regress/copy, regress/create_index, regress/triggers, regress/inherit, regress/updatable_views, regress/hash_func, regress/select, regress/select_distinct, regress/select_distinct_on, regress/select_implicit, regress/select_having, regress/subselect, regress/union, regress/join, regress/aggregates, regress/portals, regress/privileges, regress/collate, regress/matview, regress/tablesample, regress/groupingsets, regress/misc, regress/psql, regress/tsrf, regress/stats_ext, regress/rules, regress/psql_crosstab, regress/select_parallel, regress/write_parallel, regress/window, regress/functional_deps, regress/jsonb, regress/limit, regress/plpgsql, regress/rangefuncs, regress/alter_table, regress/polymorphism, regress/rowtypes, regress/largeobject, regress/with, regress/partition_join, regress/partition_aggregate, regress/stats