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
```
const auto & projection = projections_to_build[i]->projection;
        auto & projection_squash = projection_squashes[i];
        auto projection_block = projection_squash.add({});
        if (projection_block)
        {
            projection_parts[projection.name].emplace_back(
                MergeTreeDataWriter::writeTempProjectionPart(data, log, projection_block, projection, new_data_part.get(), ++block_num));
        }
    }
    const auto & projections = metadata_snapshot->projections;
    for (auto && [name, parts] : projection_parts)
    {
        LOG_DEBUG(log, "Selected {} projection_parts from {} to {}", parts.size(), parts.front()->name, parts.back()->name);
        const auto & projection = projections.get(name);
        std::map<size_t, MergeTreeData::MutableDataPartsVector> level_parts;
        size_t current_level = 0;
        size_t next_level = 1;
        level_parts[current_level] = std::move(parts);
        size_t max_parts_to_merge_in_one_level = 10;
        for (;;)
        {
            auto & current_level_parts = level_parts[current_level];
            auto & next_level_parts = level_parts[next_level];
            MergeTreeData::MutableDataPartsVector selected_parts;
            while (selected_parts.size() < max_parts_to_merge_in_one_level && !current_level_parts.empty())
            {
                selected_parts.push_back(std::move(current_level_parts.back()));
                current_level_parts.pop_back();
            }
            if (selected_parts.empty())