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

[Matt Arsenault via llvm-commits]

https://github.com/arsenm requested changes to this pull request.

This is the wrong place to handle this. AMDGPUCodeGenPrepare cannot be used for lowering. The first change should be to custom lower alloca and insert the cast there.

A follow up change should handle the alloca case in getAssumedAddrSpace to get it to fold earlier 

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