[llvm] [CodeGen] Add ValueType v3i8 (NFCI). (PR #80826)

[Matt Arsenault via llvm-commits]

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@@ -308,8 +308,11 @@ AMDGPUTargetLowering::AMDGPUTargetLowering(const TargetMachine &TM,
   setTruncStoreAction(MVT::v2f64, MVT::v2f32, Expand);
   setTruncStoreAction(MVT::v2f64, MVT::v2f16, Expand);
 
+  setTruncStoreAction(MVT::v3i32, MVT::v3i8, Expand);
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arsenm wrote:

I suspect we're missing test coverage (or we're doing unnecessary work) for the load half of this 

https://github.com/llvm/llvm-project/pull/80826