Avoid incorrectly indicating exclusion constraint wait

INSERT ... ON CONFLICT's precheck may have to wait on the outcome of
another insertion, which may or may not itself be a speculative
insertion.  This wait is not necessarily associated with an exclusion
constraint, but was always reported that way in log messages if the wait
happened to involve a tuple that had no speculative token.

Initially discovered through use of ON CONFLICT DO NOTHING, where
spurious references to exclusion constraints in log messages were more
likely.

Patch by Peter Geoghegan.
Reviewed by Julien Rouhaud.

Back-patch to 9.5 where INSERT ... ON CONFLICT was added.

src/backend/executor/execIndexing.c

@@ -725,6 +725,7 @@ retry:
 	{
 		TransactionId xwait;
 		ItemPointerData ctid_wait;
+		XLTW_Oper		reason_wait;
 		Datum		existing_values[INDEX_MAX_KEYS];
 		bool		existing_isnull[INDEX_MAX_KEYS];
 		char	   *error_new;
@@ -783,13 +784,14 @@ retry:
 			  TransactionIdPrecedes(GetCurrentTransactionId(), xwait))))
 		{
 			ctid_wait = tup->t_data->t_ctid;
+			reason_wait = indexInfo->ii_ExclusionOps ?
+				XLTW_RecheckExclusionConstr : XLTW_InsertIndex;
 			index_endscan(index_scan);
 			if (DirtySnapshot.speculativeToken)
 				SpeculativeInsertionWait(DirtySnapshot.xmin,
 										 DirtySnapshot.speculativeToken);
 			else
-				XactLockTableWait(xwait, heap, &ctid_wait,
-								  XLTW_RecheckExclusionConstr);
+				XactLockTableWait(xwait, heap, &ctid_wait, reason_wait);
 			goto retry;
 		}