[Attributor] Give up early on AANoReturn::initialize

If the function is not assumed `noreturn` we should not wait for an
update to mark the call site as "may-return".

This has two kinds of consequences:
  - We have less iterations in many tests.
  - We have less deductions based on "known information" (since we ask
    earlier, point 1, and therefore assumed information is not "known"
    yet).
The latter is an artifact that we might want to tackle properly at some
point but which is not easily fixable right now.