[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:

The actual codegen treatment appears to be correct. For SP3 compatibility we could accept the mad names on the targets where it's really an FMA, I suppose. Is there a reason to do it? 

The ISA made a grave error, and SP3 made it worse 

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