FULL OUTER JOIN get correct sargable condition

Consider

select * 

from 

  t1 full outer join t2 on on_cond

where

  where_cond

Here, we can use on_cond to construct table quick selects. We can't use where_cond with FULL OUTER JOIN.

get_sargable_cond() implements this logic for LEFT JOIN. Need to modify it for FULL OUTER JOIN.