This fixes f5b275a2 where the intent was to never produce
top-level ticked strings.
The meaning of the top_lvl parameter here doesn't align
with the intent, as explained in a new comment.
As a side note, I don't know where we guarantee that the
string literals get floated to the very top. I think that's
the only kind of floating that the simplifier does, but I
couldn't find any comment on that.