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/nodes/nodeFuncs.c

@@ -3047,6 +3047,14 @@ raw_expression_tree_walker(Node *node,
 				/* operator name is deemed uninteresting */
 			}
 			break;
+		case T_BoolExpr:
+			{
+				BoolExpr   *expr = (BoolExpr *) node;
+
+				if (walker(expr->args, context))
+					return true;
+			}
+			break;
 		case T_ColumnRef:
 			/* we assume the fields contain nothing interesting */
 			break;