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
```
{
        auto input = std::make_unique<MergeTreeSequentialSource>(
            data, metadata_snapshot, part, merging_column_names, read_with_direct_io, true);
        input->setProgressCallback(
            MergeProgressCallback(merge_entry, watch_prev_elapsed, horizontal_stage_progress));
        Pipe pipe(std::move(input));
        if (metadata_snapshot->hasSortingKey())
        {
            pipe.addSimpleTransform([&metadata_snapshot](const Block & header)
            {
                return std::make_shared<ExpressionTransform>(header, metadata_snapshot->getSortingKey().expression);
            });
        }
        pipes.emplace_back(std::move(pipe));
    }
    Names sort_columns = metadata_snapshot->getSortingKeyColumns();
    SortDescription sort_description;
    size_t sort_columns_size = sort_columns.size();
    sort_description.reserve(sort_columns_size);
    Names partition_key_columns = metadata_snapshot->getPartitionKey().column_names;
    Block header = pipes.at(0).getHeader();
    for (size_t i = 0; i < sort_columns_size; ++i)
        sort_description.emplace_back(header.getPositionByName(sort_columns[i]), 1, 1);
    
    
    
    ProcessorPtr merged_transform;
    
    bool blocks_are_granules_size = (chosen_merge_algorithm == MergeAlgorithm::Vertical);
    UInt64 merge_block_size = data_settings->merge_max_block_size;