[llvm] AMDGPU: Ensure both wavesize features are not set (PR #159234)

[Matt Arsenault via llvm-commits]

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@@ -0,0 +1,23 @@
+// RUN: llvm-mc -triple=amdgcn -mcpu=gfx1250 -mattr=+wavefrontsize64 -o - %s | FileCheck -check-prefix=GFX1250 %s
+// RUN: llvm-mc -triple=amdgcn -mcpu=gfx900 -mattr=+wavefrontsize32 -o - %s | FileCheck -check-prefix=GFX900 %s
+
+// Both sure setting both modes is supported at the same time.
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arsenm wrote:

```suggestion
// Make sure setting both modes is supported at the same time.
```

https://github.com/llvm/llvm-project/pull/159234