[llvm] [Attributor] Avoid AS-cast for function pointers (PR #65825)

[Alexander Richardson via llvm-commits]

https://github.com/arichardson commented:

This is IMO more correct that the previous hardcoded AS0, but since I am not familiar with the optimization I will defer to @arsenm 

https://github.com/llvm/llvm-project/pull/65825