[llvm] [AMDGPU][DAG] Allow extract_subvector for free register copy (PR #214152)

[Matt Arsenault via llvm-commits]

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@@ -1642,6 +1654,13 @@ SDValue AMDGPUTargetLowering::LowerEXTRACT_SUBVECTOR(SDValue Op,
   EVT VT = Op.getValueType();
   EVT SrcVT = Op.getOperand(0).getValueType();
 
+  // A free extract is a sub register reference, which isel turns into an
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arsenm wrote:

Should we even still have the custom lowering? We probably don't want to hit this for any legal types 

https://github.com/llvm/llvm-project/pull/214152