Fix counting of parrallel nodes.

Commit 2157a61 adds test which fails
with in-progress explain patch by Maxim Milyutin. During
summarization it could lost previously collected values.

Author: feodor

src/backend/executor/instrument.c

@@ -161,8 +161,8 @@ InstrAggNode(Instrumentation *dst, Instrumentation *add)
 	dst->total += add->total;
 	dst->ntuples += add->ntuples;
 	dst->nloops += add->nloops;
-	dst->nfiltered1 += add->nfiltered1;
-	dst->nfiltered2 += add->nfiltered2;
+	dst->nfiltered1 += add->nfiltered1 + add->accum_nfiltered1;
+	dst->nfiltered2 += add->nfiltered2 + add->accum_nfiltered2;
 
 	/* Add delta of buffer usage since entry to node's totals */
 	if (dst->need_bufusage)