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/util/clauses.c

@@ -3447,12 +3447,15 @@ simplify_or_arguments(List *args,
 	List	   *unprocessed_args;
 
 	/*
-	 * Since the parser considers OR to be a binary operator, long OR lists
-	 * become deeply nested expressions.  We must flatten these into long
-	 * argument lists of a single OR operator.  To avoid blowing out the stack
-	 * with recursion of eval_const_expressions, we resort to some tenseness
-	 * here: we keep a list of not-yet-processed inputs, and handle flattening
-	 * of nested ORs by prepending to the to-do list instead of recursing.
+	 * We want to ensure that any OR immediately beneath another OR gets
+	 * flattened into a single OR-list, so as to simplify later reasoning.
+	 *
+	 * To avoid stack overflow from recursion of eval_const_expressions, we
+	 * resort to some tenseness here: we keep a list of not-yet-processed
+	 * inputs, and handle flattening of nested ORs by prepending to the to-do
+	 * list instead of recursing.  Now that the parser generates N-argument
+	 * ORs from simple lists, this complexity is probably less necessary than
+	 * it once was, but we might as well keep the logic.
 	 */
 	unprocessed_args = list_copy(args);
 	while (unprocessed_args)