You have to provide the following information whenever possible.

**Describe the bug**
This query fails with the latest stable version 21.5.5.12. It worked with the last stable version 21.4.7.3.

**Does it reproduce on recent release?**
yes

**How to reproduce**
21.5.5.12

```
SELECT
    label,
    number
FROM
(
    SELECT
        'a' AS label,
        number
    FROM
    (
        SELECT number
        FROM numbers(10)
    )
    UNION ALL
    SELECT
        'b' AS label,
        number
    FROM
    (
        SELECT number
        FROM numbers(10)
    )
)
WHERE number IN
(
    SELECT number
    FROM numbers(5)
)
```

**Expected behavior**
Query should run without errors

**Error message and/or stacktrace**
```
Received exception from server (version 21.5.5):
Code: 49. DB::Exception: Received from localhost:9000. DB::Exception: Block structure mismatch in Pipe::unitePipes stream: different names of columns:
label String String(size = 0), number UInt64 UInt64(size = 0), 'b' String Const(size = 0, String(size = 1))
label String String(size = 0), number UInt64 UInt64(size = 0), 'a' String Const(size = 0, String(size = 1)).
```

bool needed_by_split_node = false;
        bool visited = false;
        bool used_in_result = false;
        
        
        Node * to_second = nullptr;
        Node * to_first = nullptr;
    };
    std::stack<Frame> stack;
    std::unordered_map<const Node *, Data> data;
    for (const auto & node : index)
        data[node].used_in_result = true;
    
    for (const auto & node : nodes)
    {
        if (split_nodes.count(&node) == 0)
            continue;
        auto & cur_data = data[&node];
        if (cur_data.needed_by_split_node)
            continue;
        cur_data.needed_by_split_node = true;
        stack.push({.node = &node});
        while (!stack.empty())
        {
            auto & cur_node = stack.top().node;
            stack.pop();
            for (const auto * child : cur_node->children)
            {
                auto & child_data = data[child];
                if (!child_data.needed_by_split_node)