This is a partial cleanup to centralize the initialization and update
decisions for AAs. Lifting the burdon and boilerplate on users and
making it harder to accidentally perform unsound deductions.
The two static helpers show how we can lift the decisions to generate an
AA into the Attributor, avoiding trivial AAs that just cost us compile
time and maintenance code (to check for pre-conditions).
Similar to loads, PHIs can be used to introduce non-dynamically unique
values into the simplification "algorithm". We need to check that PHIs
do not carry such a value from one iteration into the next as can cause
downstream reasoning to fail, e.g., downstream could think a comparison
is equal because the simplified values are equal while they are defined
in different loop iterations. Similarly, instructions in cycles are now
conservatively treated as non-dynamically unique. We could do better but
I'll leave that for the future.
The change in AAUnderlyingObjects allows us to ignore dynamically unique
when we simply look for underlying objects. The user of that AA should
be aware that the result might not be a dynamically unique value.
There is potential for endless recursion if we try to determine the
underlying objects of a load, just to end up with the load as underlying
object. A proper solution will require us to pass a visited set around.
This will happen as we cleanup genericValueTraversal soon.
