[Attributor] Avoid REQUIRED dependences in favor of OPTIONAL ones

[Attributor] Avoid REQUIRED dependences in favor of OPTIONAL ones

When we use information only to short-cut deduction or improve it, we
can use OPTIONAL dependences instead of REQUIRED ones to avoid cascading
pessimistic fixpoints.

We also need to track dependences only when we use assumed information,
e.g., we act on assumed liveness information.