Support flattening of empty-FROM subqueries and one-row VALUES tables.

We can't handle this in the general case due to limitations of the
planner's data representations; but we can allow it in many useful cases,
by being careful to flatten only when we are pulling a single-row subquery
up into a FROM (or, equivalently, inner JOIN) node that will still have at
least one remaining relation child.  Per discussion of an example from
Kyotaro Horiguchi.

src/backend/nodes/outfuncs.c

@@ -1762,6 +1762,7 @@ _outPlannerInfo(StringInfo str, const PlannerInfo *node)
 	WRITE_BOOL_FIELD(hasInheritedTarget);
 	WRITE_BOOL_FIELD(hasJoinRTEs);
 	WRITE_BOOL_FIELD(hasLateralRTEs);
+	WRITE_BOOL_FIELD(hasDeletedRTEs);
 	WRITE_BOOL_FIELD(hasHavingQual);
 	WRITE_BOOL_FIELD(hasPseudoConstantQuals);
 	WRITE_BOOL_FIELD(hasRecursion);