[Do not merge] Bug with `default-assigns`

[rachitnigam]

The new interface of seq_mem uses a content_en signal to express usage and write_en to signal a write. This means that when reading, you only need to set content_en and can leave out write_en. However, this can be problematic because it might leave write_en unassigned to (#1919).

default-assign is meant to fix this but there still might be problems with this pass. I think PIFO might have an example problem here. Pushing this change to try out icarus run of the PIFO examples.