[Attributor] Fix AANoUndef initialization

When the associated value is undef, we immediately forced to indicate a pessimistic fixpoint so far.
This patch changes the initialization to check the attribute given in IR at first and to indicate an optimistic fixpoint when it is given.
This change will enable us to catch , for example, the following case in AAUB.
```
call void @foo(i32 noundef undef)
```

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D86983