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

[Pierre van Houtryve 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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Pierre-vh wrote:

I personally think it's less confusing if we take the `addrspace(3) null` (`0xFFFFFFFF`) and use that to emit the NULL barrier, than if we use the pointer value 0 (which may be a valid LDS address IIRC).

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