[Attributor][FIX] Ensure loop PHI replacements are dynamically unique

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.