[llvm] [IR] Add `alwaysuniform` function attribute (PR #214644)

[Reem Elkhouly via llvm-commits]

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@@ -2377,6 +2377,11 @@ define void @f() "no-sse" { ... }
     would normally be considered a source of divergence; setting this attribute
     on a function means that a call to it is not a source of divergence.
 
+`alwaysuniform`
+:   A call to this function is always uniform. In uniformity analysis, a *uniform*
+    value is a value that is the same across all threads in a subgroup. Setting this
+    attribute on a function means that a call to it always returns a uniform result.
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amd-relkhoul wrote:

Correct! 
Even if the inputs are divergent, the call is guaranteed to return a uniform result.
I'll edit the description to stress on that and remove the ambiguity.

Thanks

https://github.com/llvm/llvm-project/pull/214644