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

[Shilei Tian via llvm-commits]

shiltian wrote:

> This is the wrong place to handle this. AMDGPUCodeGenPrepare cannot be used for lowering.

Then I wonder what `AMDGPUCodeGenPrepareImpl::visitAddrSpaceCastInst` is?

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