Avoid creating PlaceHolderVars immediately within PlaceHolderVars.

Such a construction is useless since the lower PlaceHolderVar is already
nullable; no need to make it more so.  Noted while pursuing bug #6154.

This is just a minor planner efficiency improvement, since the final plan
will come out the same anyway after PHVs are flattened.  So not worth the
risk of back-patching.

src/backend/optimizer/prep/prepjointree.c

@@ -1411,6 +1411,12 @@ pullup_replace_vars_callback(Var *var,
 				/* Simple Vars always escape being wrapped */
 				wrap = false;
 			}
+			else if (newnode && IsA(newnode, PlaceHolderVar) &&
+					 ((PlaceHolderVar *) newnode)->phlevelsup == 0)
+			{
+				/* No need to wrap a PlaceHolderVar with another one, either */
+				wrap = false;
+			}
 			else if (rcon->wrap_non_vars)
 			{
 				/* Wrap all non-Vars in a PlaceHolderVar */
@@ -1420,10 +1426,16 @@ pullup_replace_vars_callback(Var *var,
 			{
 				/*
 				 * If it contains a Var of current level, and does not contain
-				 * any non-strict constructs, then it's certainly nullable and
-				 * we don't need to insert a PlaceHolderVar.  (Note: in future
-				 * maybe we should insert PlaceHolderVars anyway, when a tlist
-				 * item is expensive to evaluate?
+				 * any non-strict constructs, then it's certainly nullable so
+				 * we don't need to insert a PlaceHolderVar.
+				 *
+				 * This analysis could be tighter: in particular, a non-strict
+				 * construct hidden within a lower-level PlaceHolderVar is not
+				 * reason to add another PHV.  But for now it doesn't seem
+				 * worth the code to be more exact.
+				 *
+				 * Note: in future maybe we should insert a PlaceHolderVar
+				 * anyway, if the tlist item is expensive to evaluate?
 				 */
 				if (contain_vars_of_level((Node *) newnode, 0) &&
 					!contain_nonstrict_functions((Node *) newnode))