[llvm] [AMDGPU] Simplify and document waitcnt handling on call and return (PR #172453)

[Jay Foad via llvm-commits]

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@@ -2128,6 +2125,14 @@ The AMDGPU backend supports the following calling conventions:
 
      =============================== ==========================================================
 
+The following ABI conventions apply to all calling conventions that are used for
+callable functions (i.e. those that do not correspond to hardware entry points):
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jayfoad wrote:

Yeah, my hesitation is that the rules for that are imposed by hardware, so they really belong in hardware documentation not here. But I guess I can try to say something helpful anyway.

https://github.com/llvm/llvm-project/pull/172453