[llvm] [NVPTX] Lower invalid `ISD::ADDRSPACECAST` (PR #125607)

[Justin Fargnoli via llvm-commits]

justinfargnoli wrote:

Thank you all for your comments. I agree that my initial approach is not the best way forward. 

In the meantime, I've modified the PR to custom lower invalid `ISD::ADDRSPACECAST`s to `ISD::UNDEF`. 

I do agree that:
> Ideally we'd do it as soon as we create such an invalid ASC, but for that we must have target info available.

However, I'm unsure of the best place to do this. e.g., `InstCombine` would not want to contain this optimization because it's target-dependent, correct? 

https://github.com/llvm/llvm-project/pull/125607