Optimize COALESCE(l.x, r.y) from l left join r on l.x = r.y to l.x

[kaikalur]

Some tool generated queries do weird things like:

COALESCE(l.x, r.y) from l left join r on l.x = r.y

which is simply l.x. By using coalesce, the plan gets messed up sometimes especially bucketed joins. So we should optimize this.

CC: @feilong-liu @maniloya