[llvm] AMDGPU: Add baseline tests for simplify elts of readfirstlane (PR #128645)

[Matt Arsenault via llvm-commits]

arsenm wrote:

* **#128648** <a href="https://app.graphite.dev/github/pr/llvm/llvm-project/128648?utm_source=stack-comment-icon" target="_blank"><img src="https://static.graphite.dev/graphite-32x32-black.png" alt="Graphite" width="10px" height="10px"/></a>
* **#128647** <a href="https://app.graphite.dev/github/pr/llvm/llvm-project/128647?utm_source=stack-comment-icon" target="_blank"><img src="https://static.graphite.dev/graphite-32x32-black.png" alt="Graphite" width="10px" height="10px"/></a>
* **#128646** <a href="https://app.graphite.dev/github/pr/llvm/llvm-project/128646?utm_source=stack-comment-icon" target="_blank"><img src="https://static.graphite.dev/graphite-32x32-black.png" alt="Graphite" width="10px" height="10px"/></a>
* **#128645** <a href="https://app.graphite.dev/github/pr/llvm/llvm-project/128645?utm_source=stack-comment-icon" target="_blank"><img src="https://static.graphite.dev/graphite-32x32-black.png" alt="Graphite" width="10px" height="10px"/></a> 👈 <a href="https://app.graphite.dev/github/pr/llvm/llvm-project/128645?utm_source=stack-comment-view-in-graphite" target="_blank">(View in Graphite)</a>
* `main`


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https://github.com/llvm/llvm-project/pull/128645