[llvm] [NVPTX] Add syncscope support for cmpxchg (PR #140812)

[via llvm-commits]

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@@ -174,7 +174,8 @@ enum Scope : ScopeUnderlyingType {
   Cluster = 2,
   Device = 3,
   System = 4,
-  LASTSCOPE = System
+  DefaultDevice = 5,
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gonzalobg wrote:

```suggestion
  DefaultDevice = 5, //  For SM < 70: denotes PTX op implicit/default .gpu scope
```

https://github.com/llvm/llvm-project/pull/140812