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

[Matt Arsenault via llvm-commits]

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@@ -0,0 +1,16 @@
+; RUN: opt -mtriple amdgcn-- -passes='print<uniformity>' -disable-output %s 2>&1 | FileCheck %s
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arsenm wrote:

```suggestion
; RUN: opt -mtriple=amdgpu-- -passes='print<uniformity>' -disable-output %s 2>&1 | FileCheck %s
```

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