[llvm] [AMDGPU] Fix RewriteMFMAFormSchedStage (PR #193429)

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+# XFAIL: *
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xgxanq wrote:

> XFAIL?

I don't understand. arsenm let me add tests first.
<img width="1276" height="523" alt="image" src="https://github.com/user-attachments/assets/68750f29-45bd-4585-b946-d0ad6b70412b" />
I will add my second fix commit if this first CI is passed.

https://github.com/llvm/llvm-project/pull/193429