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
```
size_t column_elems_written = 0;
            column_to.writePrefix();
            while (!merges_blocker.isCancelled() && (block = column_gathered_stream.read()))
            {
                column_elems_written += block.rows();
                column_to.write(block);
            }
            if (merges_blocker.isCancelled())
                throw Exception("Cancelled merging parts", ErrorCodes::ABORTED);
            column_gathered_stream.readSuffix();
            auto changed_checksums = column_to.writeSuffixAndGetChecksums(new_data_part, checksums_gathered_columns, need_sync);
            checksums_gathered_columns.add(std::move(changed_checksums));
            if (rows_written != column_elems_written)
            {
                throw Exception("Written " + toString(column_elems_written) + " elements of column " + column_name +
                                ", but " + toString(rows_written) + " rows of PK columns", ErrorCodes::LOGICAL_ERROR);
            }
            
            merge_entry->columns_written += 1;
            merge_entry->bytes_written_uncompressed += column_gathered_stream.getProfileInfo().bytes;
            merge_entry->progress.store(progress_before + column_sizes->columnWeight(column_name), std::memory_order_relaxed);
        }
        tmp_disk->removeFile(rows_sources_file_path);
    }
    for (const auto & part : parts)
        new_data_part->minmax_idx.merge(part->minmax_idx);
    
    {
        double elapsed_seconds = merge_entry->watch.elapsedSeconds();
        LOG_DEBUG(log,