[llvm] [AMDGPU] Add amdgpu-lower-special-lds pass to lower named-barrier LDS (PR #165692)

[Pierre van Houtryve via llvm-commits]

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+//===-- AMDGPULowerSpecialLDS.cpp -----------------------------------------===//
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Pierre-vh wrote:

Maybe wait for other reviewers to give feedback first, but I think we should name this "LowerBarrierGVs" instead.

I really don't want to see more "special LDS variables" proliferate. Making the pass specific to the barrier GV would help to not encourage others from adding another "special LDS not not really LDS" GV. 

https://github.com/llvm/llvm-project/pull/165692