[llvm] [AMDGPU] Expand constant named barrier matching in SDag (PR #191506)

[Jay Foad via llvm-commits]

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@@ -1307,6 +1307,9 @@ as the pointer operand of a regular memory access instruction or intrinsic.
 Pointers to named barrier objects are intended to be used with dedicated
 intrinsics. Reading from or writing to such pointers is undefined behavior.
 
+The NULL named barrier (needed for un-joining a barrier) is represented in IR by
+``ptr addrspace(3) nullptr``.
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jayfoad wrote:

This might be true but it's potentially confusing. I think you mean the pointer value 0x00000000 here right, not addrspace 3's "null"/"sentinel" value which is 0xFFFFFFFF.

Also:
```suggestion
``ptr addrspace(3) null``.
```

https://github.com/llvm/llvm-project/pull/191506