[llvm] [AMDGPU] Fix LDS access via flat pointer argument in amdgpu-sw-lower-lds (PR #209842)

[Shilei Tian via llvm-commits]

shiltian wrote:

How did we end up with `AS3 -> AS0 -> AS3` across the function call boundary? I understand the `AS3 -> AS0` part, since we always pass AS0 pointer across function call, but how does the `AS0 -> AS3` happen w/o any sort of `InferAddressSpace`? If we are to change the AS of a GV, we **have to** trace all its use and update all potential uses of it.

https://github.com/llvm/llvm-project/pull/209842