[llvm] [Attributor] Don't replace `AddrSpaceCast` with `ConstantPointerNull` (PR #126779)

[Shilei Tian via llvm-commits]

shiltian wrote:

Ping.

AFAICT, only `ConstantPointerNull` matters here because it represents `null` in all AS. For the others, they are not constant thus will not be replaced by this code path.

https://github.com/llvm/llvm-project/pull/126779