[llvm] [AMDGPU] selecting v_sat_pk instruction, version 2 (PR #123297)

[Matt Arsenault via llvm-commits]

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@@ -332,6 +332,8 @@ def AMDGPUumed3 : SDNode<"AMDGPUISD::UMED3", AMDGPUDTIntTernaryOp,
   []
 >;
 
+def AMDGPUsat_pk_cast : SDNode<"AMDGPUISD::SAT_PK_CAST", SDTUnaryOp, []>;
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arsenm wrote:

Explain what the node is, not just to avoid v2i8. It's to pack a v2i18 into i16 

https://github.com/llvm/llvm-project/pull/123297