Avoid recursion when processing simple lists of AND'ed or OR'ed clauses.

Since most of the system thinks AND and OR are N-argument expressions
anyway, let's have the grammar generate a representation of that form when
dealing with input like "x AND y AND z AND ...", rather than generating
a deeply-nested binary tree that just has to be flattened later by the
planner.  This avoids stack overflow in parse analysis when dealing with
queries having more than a few thousand such clauses; and in any case it
removes some rather unsightly inconsistencies, since some parts of parse
analysis were generating N-argument ANDs/ORs already.

It's still possible to get a stack overflow with weirdly parenthesized
input, such as "x AND (y AND (z AND ( ... )))", but such cases are not
mainstream usage.  The maximum depth of parenthesization is already
limited by Bison's stack in such cases, anyway, so that the limit is
probably fairly platform-independent.

Patch originally by Gurjeet Singh, heavily revised by me

src/backend/optimizer/prep/prepjointree.c

@@ -133,9 +133,9 @@ static Node *find_jointree_node_for_rel(Node *jtnode, int relid);
  * transformations if any are found.
  *
  * This routine has to run before preprocess_expression(), so the quals
- * clauses are not yet reduced to implicit-AND format.  That means we need
- * to recursively search through explicit AND clauses, which are
- * probably only binary ANDs.  We stop as soon as we hit a non-AND item.
+ * clauses are not yet reduced to implicit-AND format, and are not guaranteed
+ * to be AND/OR-flat either.  That means we need to recursively search through
+ * explicit AND clauses.  We stop as soon as we hit a non-AND item.
  */
 void
 pull_up_sublinks(PlannerInfo *root)