[llvm] AMDGPU: Preliminary documentation for named barriers (PR #165502)

[Pierre van Houtryve via llvm-commits]

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@@ -6621,6 +6668,138 @@ Multiple tags can be used at the same time to synchronize with more than one add
   better code optimization, at the cost of synchronizing additional address
   spaces.
 
+.. _amdgpu-memory-model-barriers:
+
+Hardware Barriers
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Pierre-vh wrote:

I'm hoping to land the formal model soon(-ish), so maybe this section won't be necessary 

https://github.com/llvm/llvm-project/pull/165502