Can't combine zlib processed field with unprocessed field in instance
@GreyCat as requested, here's an issue for this bug

```yaml
seq:
  - id: buf_uncomp
    size: meh
    if: is_uncompressed
  - id: buf_comp
    size: meh
    process: zlib
    if: not is_uncompressed
instances:
  buf:
    value: is_uncompressed ? buf_comp : buf_uncomp
```

Results in this error:
```
can't combine output types:
BytesLimitType(Name(identifier(length)),None,false,None,Some(ProcessZlib))
vs
BytesLimitType(Name(identifier(length)),None,false,None,None)
