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
```
merged_transform = std::make_unique<VersionedCollapsingTransform>(
                header, pipes.size(), sort_description, merging_params.sign_column,
                merge_block_size, rows_sources_write_buf.get(), blocks_are_granules_size);
            break;
    }
    QueryPipeline pipeline;
    pipeline.init(Pipe::unitePipes(std::move(pipes)));
    pipeline.addTransform(std::move(merged_transform));
    pipeline.setMaxThreads(1);
    BlockInputStreamPtr merged_stream = std::make_shared<PipelineExecutingBlockInputStream>(std::move(pipeline));
    if (deduplicate)
        merged_stream = std::make_shared<DistinctSortedBlockInputStream>(merged_stream, sort_description, SizeLimits(), 0 , deduplicate_by_columns);
    if (need_remove_expired_values)
        merged_stream = std::make_shared<TTLBlockInputStream>(merged_stream, data, metadata_snapshot, new_data_part, time_of_merge, force_ttl);
    if (metadata_snapshot->hasSecondaryIndices())
    {
        const auto & indices = metadata_snapshot->getSecondaryIndices();
        merged_stream = std::make_shared<ExpressionBlockInputStream>(
            merged_stream, indices.getSingleExpressionForIndices(metadata_snapshot->getColumns(), data.getContext()));
        merged_stream = std::make_shared<MaterializingBlockInputStream>(merged_stream);
    }
    const auto & index_factory = MergeTreeIndexFactory::instance();
    MergedBlockOutputStream to{
        new_data_part,
        metadata_snapshot,
        merging_columns,
        index_factory.getMany(metadata_snapshot->getSecondaryIndices()),
        compression_codec,
        blocks_are_granules_size};
    merged_stream->readPrefix();