[llvm] [AMDGPU][MC]FMA-mix instructions should not be supported in GFX908 etc. (PR #109873)

[Matt Arsenault via llvm-commits]

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@@ -1426,7 +1425,6 @@ def FeatureISAVersion9_4_Common : FeatureSet<
   [FeatureGFX9,
    FeatureGFX90AInsts,
    FeatureGFX940Insts,
-   FeatureFmaMixInsts,
----------------
arsenm wrote:

...How did this happen? gfx906 replaced mad_mix with fma_mix, and then all the MI went back to mad_mix? 

Looking at the features lists, I'm not sure mad_mix is properly added. Can you confirm that we have test coverage for the mad_mix cases for 908/90a/940? 

https://github.com/llvm/llvm-project/pull/109873