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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> This PR extends the pass use-tracing to follow LDS that arrives through a flat pointer argument and redirects that access into the global backing buffer.

This isn't a viable strategy though. You can't rely on tracing back to find the original object. A proper lowering needs to independently and completely replace the definition of the object 

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