[Attributor] Remove now obsolete initialization code

With the helpers in place to judge AAs [1] we can remove the custom
rolled initialization checking code. This exposed a minor oversight in
the AAMemoryLocation where we did not check the IR before we gave up for
a declaration.

[1] https://github.com/llvm/llvm-project/commit/d33bca840a824f59296230e83181012a0b1ba7ec