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
```
stages.emplace_back(context);
            if (stages.size() == 1) 
                stages.emplace_back(context);
            for (const auto & column : changed_columns)
                stages.back().column_to_updated.emplace(
                    column, std::make_shared<ASTIdentifier>(column));
        }
        if (!unchanged_columns.empty())
        {
            if (!stages.empty())
            {
                std::vector<Stage> stages_copy;
                
                for (const auto & stage : stages)
                {
                    stages_copy.emplace_back(context);
                    stages_copy.back().column_to_updated = stage.column_to_updated;
                    stages_copy.back().output_columns = stage.output_columns;
                    stages_copy.back().filters = stage.filters;
                }
                const ASTPtr select_query = prepareInterpreterSelectQuery(stages_copy,  true);
                InterpreterSelectQuery interpreter{
                    select_query, context, storage, metadata_snapshot,
                    SelectQueryOptions().analyze( false).ignoreLimits()};
                auto first_stage_header = interpreter.getSampleBlock();
                QueryPlan plan;
                auto source = std::make_shared<NullSource>(first_stage_header);
                plan.addStep(std::make_unique<ReadFromPreparedSource>(Pipe(std::move(source))));
                auto pipeline = addStreamsForLaterStages(stages_copy, plan);
                updated_header = std::make_unique<Block>(pipeline->getHeader());