[DA] Fix function `unifySubscriptType`

[sjoerdmeijer]

This is extracted from #160924 and little bit of a placeholder (more details will follow), but this is an attempt to document all know DA issues.

The complaint about function unifySubscriptType was:

Maybe this should be applied recursively to an AddRec, after we implement the appropriate handling for wrap of an AddRec

I will add this to the meta issue #161805 that lists all DA issues.

[sjoerdmeijer]

CC: @kasuga-fj