[Attributor][FIX] Handle function pointers properly in AANonNull

[Attributor][FIX] Handle function pointers properly in AANonNull

Before we tired to create a dominator tree for a declaration when we
wanted to determine if the function pointer is `nonnull`. We now avoid
looking at global values if `Value::getPointerDereferenceableBytes` not
already determined `nonnull`.