[Attributor][FIX] Improve care when dealing with liveness

This patch adds two checks that have in experiments caused issues. One
was an oversight that allowed new AAs during cleanup to be optimistic.
The other treated functions as functions even if they were used as
values, e.g., in a cast instruction. In such cases we might have assumed
the value is dead if the function is not entered, which isn't true.

The new test functions don't expose a bug but I kept them around.