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 String & column_name = it_name_and_type->name;
            Names column_names{column_name};
            Float64 progress_before = merge_entry->progress.load(std::memory_order_relaxed);
            MergeStageProgress column_progress(progress_before, column_sizes->columnWeight(column_name));
            for (size_t part_num = 0; part_num < parts.size(); ++part_num)
            {
                auto column_part_source = std::make_shared<MergeTreeSequentialSource>(
                    data, metadata_snapshot, parts[part_num], column_names, read_with_direct_io, true);
                column_part_source->setProgressCallback(
                    MergeProgressCallback(merge_entry, watch_prev_elapsed, column_progress));
                QueryPipeline column_part_pipeline;
                column_part_pipeline.init(Pipe(std::move(column_part_source)));
                column_part_pipeline.setMaxThreads(1);
                column_part_streams[part_num] =
                        std::make_shared<PipelineExecutingBlockInputStream>(std::move(column_part_pipeline));
            }
            rows_sources_read_buf.seek(0, 0);
            ColumnGathererStream column_gathered_stream(column_name, column_part_streams, rows_sources_read_buf);
            MergedColumnOnlyOutputStream column_to(
                new_data_part,
                metadata_snapshot,
                column_gathered_stream.getHeader(),
                compression_codec,
                
                
                
                std::vector<MergeTreeIndexPtr>{},
                &written_offset_columns,
                to.getIndexGranularity());