Fix "cannot accept a set" error when only some arms of a CASE return a set.

In commit c1352052ef, I implemented an
optimization that assumed that a function's argument expressions would
either always return a set (ie multiple rows), or always not.  This is
wrong however: we allow CASE expressions in which some arms return a set
of some type and others just return a scalar of that type.  There may be
other examples as well.  To fix, replace the run-time test of whether an
argument returned a set with a static precheck (expression_returns_set).
This adds a little bit of query startup overhead, but it seems barely
measurable.

Per bug #8228 from David Johnston.  This has been broken since 8.0,
so patch all supported branches.

src/test/regress/expected/rangefuncs.out

@@ -930,3 +930,17 @@ select * from foobar();  -- fail
 ERROR:  function return row and query-specified return row do not match
 DETAIL:  Returned row contains 3 attributes, but query expects 2.
 drop function foobar();
+-- check behavior when a function's input sometimes returns a set (bug #8228)
+SELECT *,
+  lower(CASE WHEN id = 2 THEN (regexp_matches(str, '^0*([1-9]\d+)$'))[1]
+        ELSE str
+        END)
+FROM
+  (VALUES (1,''), (2,'0000000049404'), (3,'FROM 10000000876')) v(id, str);
+ id |       str        |      lower       
+----+------------------+------------------
+  1 |                  | 
+  2 | 0000000049404    | 49404
+  3 | FROM 10000000876 | from 10000000876
+(3 rows)
+

src/test/regress/sql/rangefuncs.sql

@@ -455,3 +455,12 @@ $$ select (1, 2.1, 3) $$ language sql;
 select * from foobar();  -- fail
 
 drop function foobar();
+
+-- check behavior when a function's input sometimes returns a set (bug #8228)
+
+SELECT *,
+  lower(CASE WHEN id = 2 THEN (regexp_matches(str, '^0*([1-9]\d+)$'))[1]
+        ELSE str
+        END)
+FROM
+  (VALUES (1,''), (2,'0000000049404'), (3,'FROM 10000000876')) v(id, str);