[llvm] AMDGPU/GlobalISel: Add test for fma_mix with source from unmerge (PR #102129)

[Petar Avramovic via llvm-commits]

petar-avramovic wrote:

* **#102130** <a href="https://app.graphite.dev/github/pr/llvm/llvm-project/102130?utm_source=stack-comment-icon" target="_blank"><img src="https://static.graphite.dev/graphite-32x32-black.png" alt="Graphite" width="10px" height="10px"/></a>
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https://github.com/llvm/llvm-project/pull/102129