[llvm] [AMDGPU] Support alloca in AS0 (PR #136584)

[Shilei Tian via llvm-commits]

shiltian wrote:

Based on the feedback in https://github.com/llvm/llvm-project/pull/136865 and https://github.com/llvm/llvm-project/pull/135820, I think not supporting this is the more appropriate direction. I propose that we close this PR and instead enforce that `alloca` must be in AS5 in the verifier, basically reverting to what https://github.com/llvm/llvm-project/pull/135820 initially did.

This way, we avoid hitting backend errors like "cannot select frameindex"” which can be misleading and make it sound like a backend bug when it actually isn't. If we find more practical use cases in the future that require supporting AS0, we can always revisit and reopen this PR.

What do you think? @arsenm

https://github.com/llvm/llvm-project/pull/136584