Postpone some more stuff out of ExecInitModifyTable.

Delay creation of the projections for INSERT and UPDATE tuples
until they're needed.  This saves a pretty fair amount of work
when only some of the partitions are actually touched.

The logic associated with identifying junk columns in UPDATE/DELETE
is moved to another loop, allowing removal of one loop over the
target relations; but it didn't actually change at all.

Extracted from a larger patch, which seemed to me to be too messy
to push in one commit.

Amit Langote, reviewed at different times by Heikki Linnakangas and
myself

Discussion: https://postgr.es/m/CA+HiwqG7ZruBmmih3wPsBZ4s0H2EhywrnXEduckY5Hr3fWzPWA@mail.gmail.com

src/backend/executor/execMain.c

@@ -1221,6 +1221,7 @@ InitResultRelInfo(ResultRelInfo *resultRelInfo,
 	resultRelInfo->ri_projectNew = NULL;
 	resultRelInfo->ri_newTupleSlot = NULL;
 	resultRelInfo->ri_oldTupleSlot = NULL;
+	resultRelInfo->ri_projectNewInfoValid = false;
 	resultRelInfo->ri_FdwState = NULL;
 	resultRelInfo->ri_usesFdwDirectModify = false;
 	resultRelInfo->ri_ConstraintExprs = NULL;

src/include/nodes/execnodes.h

@@ -431,6 +431,8 @@ typedef struct ResultRelInfo
 	TupleTableSlot *ri_newTupleSlot;
 	/* Slot to hold the old tuple being updated */
 	TupleTableSlot *ri_oldTupleSlot;
+	/* Have the projection and the slots above been initialized? */
+	bool		ri_projectNewInfoValid;
 
 	/* triggers to be fired, if any */
 	TriggerDesc *ri_TrigDesc;