Fix wrong number of maxAttrNum in TupleSplitState (#14927)

we cannot get corrent number of projecting targets for executing TupleSplit Node
if wrong maxAttrNum in TupleSplitState, that cause wrong results in multi-dqa sql.
```
select count(distinct a), count(distinct b) from dqa_f4 group by c;
```
For each splited tuple, we also need to project column `c` as group column besides
distinct column `a` and `b`. As a result, toal maxAttrNum of TupleSplit is three instead of two,
which also decided totals projection columns of Node Tuplesplit.
To figure maxAttrNum correctly, we need to calculate again after we initiated all elements
in TupleSplitState.

src/backend/executor/nodeTupleSplit.c

@@ -103,8 +103,6 @@ ExecInitTupleSplit(TupleSplit *node, EState *estate, int eflags)
 		i ++;
 	}
 
-	tup_spl_state->maxAttrNum = maxAttrNum;
-
 	/*
 	 * fetch group by expr bitmap set
 	 */
@@ -146,6 +144,15 @@ ExecInitTupleSplit(TupleSplit *node, EState *estate, int eflags)
 		bms_free(orig_bms);
 	}
 
+	/*
+	 * Update maxAttrNum which is used to calculate projection number
+	 * of ExecTupleSplit
+	 */
+	int x = bms_prev_member(skip_split_bms, -1);
+	if (x > maxAttrNum)
+		maxAttrNum = x;
+	tup_spl_state->maxAttrNum = maxAttrNum;
+
 	bms_free(skip_split_bms);
 
 	return tup_spl_state;