Use fast path in plpgsql's RETURN/RETURN NEXT in more cases.

 Use fast path in plpgsql's RETURN/RETURN NEXT in more cases.
 
 exec_stmt_return() and exec_stmt_return_next() have fast-path code for
 handling a simple variable reference (i.e. "return var") without going
 through the full expression evaluation machinery.  For some reason,
 pl_gram.y was under the impression that this fast path only applied for
 record/row variables; but in reality code for handling regular scalar
 variables has been there all along.  Adjusting the logic to allow that
 code to be used actually results in a net savings of code in pl_gram.y
 (by eliminating some redundancy), and it buys a measurable though not
 very impressive amount of speedup.
 
 Noted while fooling with my expanded-array patch, wherein this makes a much
 bigger difference because it enables returning an expanded array variable
 without an extra flattening step.  But AFAICS this is a win regardless,
 so commit it separately.