[Attributor] Introduce an optimistic getUnderlyingObjects helper

As the `llvm::getUnderlyingObjects` helper, the optimistic version
collects objects that might be the base of a given pointer. In contrast
to the llvm variant, the optimistic one will use assumed information,
e.g., about select conditions or dead blocks, to provide a more precise
result.

Differential Revision: https://reviews.llvm.org/D103859