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

[Petar Avramovic via llvm-commits]

petar-avramovic wrote:

### Merge activity

* **Aug 7, 6:06 AM EDT**: @petar-avramovic started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/102129).


https://github.com/llvm/llvm-project/pull/102129