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
```
FutureMergedMutatedPart & future_part,
    bool aggressive,
    size_t max_total_size_to_merge,
    const AllowedMergingPredicate & can_merge_callback,
    bool merge_with_ttl_allowed,
    String * out_disable_reason)
{
    MergeTreeData::DataPartsVector data_parts = data.getDataPartsVector();
    const auto data_settings = data.getSettings();
    auto metadata_snapshot = data.getInMemoryMetadataPtr();
    if (data_parts.empty())
    {
        if (out_disable_reason)
            *out_disable_reason = "There are no parts in the table";
        return SelectPartsDecision::CANNOT_SELECT;
    }
    time_t current_time = std::time(nullptr);
    IMergeSelector::PartsRanges parts_ranges;
    StoragePolicyPtr storage_policy = data.getStoragePolicy();