version 21.5.5.12 (official build)

I'm trying to change data type for a column that is used in the TTL expression.
It works well when the table is empty.
When the table is not empty, the mutation fails.
As a result the table ends up in an inconsistent state.

```
create table test
(
InsertionDateTime DateTime,
TTLDays Int32 DEFAULT CAST(365, 'Int32')
)
Engine=MergeTree()
order by tuple()
TTL InsertionDateTime + toIntervalDay(TTLDays);

Ok.

insert into test values (now(), 23);

Ok.

ALTER TABLE test modify column TTLDays Int16 DEFAULT CAST(365, 'Int16');

Code: 341, e.displayText() = DB::Exception: Exception happened during execution of mutation 'mutation_2.txt' with part 'all_1_1_0' reason: 'Code: 10, e.displayText() = DB::Exception: Not found column InsertionDateTime in block. There are only columns: TTLDays (version 21.5.5.12 (official build))'. This error maybe retryable or not. In case of unretryable error, mutation can be killed with KILL MUTATION query (version 21.5.5.12 (official build)) [DB Errorcode=341]

insert into test values (now(), 23);

Ok.

select table, column, type, sum(rows) rows, sum(column_bytes_on_disk) on_disk
from system.parts_columns where column like 'TTLDays'
group by database, table, column, type

table | column  | type  | rows | on_disk
------+---------+-------+------+--------
test  | TTLDays | Int32 |    1 |      86
test  | TTLDays | Int16 |    1 |      84
```
to.writePrefix();
    size_t rows_written = 0;
    const size_t initial_reservation = space_reservation ? space_reservation->getSize() : 0;
    auto is_cancelled = [&]()
    {
        return merges_blocker.isCancelled()
            || (need_remove_expired_values && ttl_merges_blocker.isCancelled())
            || merge_entry->is_cancelled.load(std::memory_order_relaxed);
    };
    Block block;
    while (!is_cancelled() && (block = merged_stream->read()))
    {
        rows_written += block.rows();
        to.write(block);
        merge_entry->rows_written = merged_stream->getProfileInfo().rows;
        merge_entry->bytes_written_uncompressed = merged_stream->getProfileInfo().bytes;
        
        if (space_reservation && sum_input_rows_upper_bound)
        {
            
            
            Float64 progress = (chosen_merge_algorithm == MergeAlgorithm::Horizontal)
                ? std::min(1., 1. * rows_written / sum_input_rows_upper_bound)
                : std::min(1., merge_entry->progress.load(std::memory_order_relaxed));
            space_reservation->update(static_cast<size_t>((1. - progress) * initial_reservation));
        }
    }
    merged_stream->readSuffix();
    merged_stream.reset();
    if (merges_blocker.isCancelled())