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
```
{
        return static_cast<Float64>(sum_index_columns) / sum_total;
    }
};
/** Progress callback.
  * What it should update:
  * - approximate progress
  * - amount of read rows
  * - various metrics
  * - time elapsed for current merge.
  */
struct MergeStageProgress
{
    explicit MergeStageProgress(Float64 weight_)
        : is_first(true) , weight(weight_)
    {
    }
    MergeStageProgress(Float64 initial_progress_, Float64 weight_)
        : initial_progress(initial_progress_), is_first(false), weight(weight_)
    {
    }
    Float64 initial_progress = 0.0;
    bool is_first;
    Float64 weight;
    UInt64 total_rows = 0;
    UInt64 rows_read = 0;
};
class MergeProgressCallback