Avoid manifesting a CTE (or other view) multiple times when it is possible to

reuse the first manifestation.

FossilOrigin-Name: 9692f510803c9b9725abb687d7c10fbc0d5ed784479ec6f3fcc55925a87fe16d

src/sqliteInt.h

@@ -3184,6 +3184,7 @@ struct Select {
 #define SF_View          0x0200000 /* SELECT statement is a view */
 #define SF_NoopOrderBy   0x0400000 /* ORDER BY is ignored for this query */
 #define SF_UpdateFrom    0x0800000 /* Statement is an UPDATE...FROM */
+#define SF_PushDown      0x1000000 /* SELECT has be modified by push-down opt */
 
 /*
 ** The results of a SELECT can be distributed in several ways, as defined