Remove bogus assertion about polymorphic SQL function result.

It is possible to reach check_sql_fn_retval() with an unresolved
polymorphic rettype, resulting in an assertion failure as demonstrated
by one of the added test cases.  However, the code following that
throws what seems an acceptable error message, so just remove the
Assert and adjust commentary.

While here, I thought it'd be a good idea to provide some parallel
tests of SQL-function and PL/pgSQL-function polymorphism behavior.
Some of these cases are perhaps duplicative of tests elsewhere,
but we hadn't any organized coverage of the topic AFAICS.

Although that assertion's been wrong all along, it won't have any
effect in production builds, so I'm not bothering to back-patch.

Discussion: https://postgr.es/m/21569.1584314271@sss.pgh.pa.us

src/test/regress/sql/plpgsql.sql

@@ -1558,6 +1558,66 @@ END;' language plpgsql;
 SELECT * FROM test_ret_rec_dyn(1500) AS (a int, b int, c int);
 SELECT * FROM test_ret_rec_dyn(5) AS (a int, b numeric, c text);
 
+--
+-- Test some simple polymorphism cases.
+--
+
+create function f1(x anyelement) returns anyelement as $$
+begin
+  return x + 1;
+end$$ language plpgsql;
+
+select f1(42) as int, f1(4.5) as num;
+select f1(point(3,4));  -- fail for lack of + operator
+
+drop function f1(x anyelement);
+
+create function f1(x anyelement) returns anyarray as $$
+begin
+  return array[x + 1, x + 2];
+end$$ language plpgsql;
+
+select f1(42) as int, f1(4.5) as num;
+
+drop function f1(x anyelement);
+
+create function f1(x anyarray) returns anyelement as $$
+begin
+  return x[1];
+end$$ language plpgsql;
+
+select f1(array[2,4]) as int, f1(array[4.5, 7.7]) as num;
+
+select f1(stavalues1) from pg_statistic;  -- fail, can't infer element type
+
+drop function f1(x anyarray);
+
+create function f1(x anyarray) returns anyarray as $$
+begin
+  return x;
+end$$ language plpgsql;
+
+select f1(array[2,4]) as int, f1(array[4.5, 7.7]) as num;
+
+select f1(stavalues1) from pg_statistic;  -- fail, can't infer element type
+
+drop function f1(x anyarray);
+
+-- fail, can't infer type:
+create function f1(x anyelement) returns anyrange as $$
+begin
+  return array[x + 1, x + 2];
+end$$ language plpgsql;
+
+create function f1(x anyrange) returns anyarray as $$
+begin
+  return array[lower(x), upper(x)];
+end$$ language plpgsql;
+
+select f1(int4range(42, 49)) as int, f1(float8range(4.5, 7.8)) as num;
+
+drop function f1(x anyrange);
+
 --
 -- Test handling of OUT parameters, including polymorphic cases.
 -- Note that RETURN is optional with OUT params; we try both ways.