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)).
```

#include <Processors/QueryPlan/UnionStep.h>
#include <Processors/QueryPipeline.h>
#include <Processors/Sources/NullSource.h>
#include <Interpreters/ExpressionActions.h>
namespace DB
{
namespace ErrorCodes
{
    extern const int LOGICAL_ERROR;
}
static Block checkHeaders(const DataStreams & input_streams)
{
    if (input_streams.empty())
        throw Exception(ErrorCodes::LOGICAL_ERROR, "Cannot unite an empty set of query plan steps");
    Block res = input_streams.front().header;
    for (const auto & stream : input_streams)
        assertBlocksHaveEqualStructure(stream.header, res, "UnionStep");
    return res;
}
UnionStep::UnionStep(DataStreams input_streams_, size_t max_threads_)