[llvm] [SelectionDAG] Add PARTIAL_REDUCE_U/SMLA ISD Nodes (PR #125207)

[Philip Reames via llvm-commits]

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@@ -3182,6 +3186,11 @@ void DAGTypeLegalizer::SplitVecRes_VP_REVERSE(SDNode *N, SDValue &Lo,
   std::tie(Lo, Hi) = DAG.SplitVector(Load, DL);
 }
 
+void DAGTypeLegalizer::SplitVecRes_PARTIAL_REDUCE_MLA(SDNode *N) {
+  SDValue Res = TLI.expandPartialReduceMLA(N, DAG);
+  ReplaceValueWith(SDValue(N, 0), Res);
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preames wrote:

Based on the precedent of SplitVecRes_VECTOR_SPLICE - which is the only example of expanding during SplitVecRes I found - I think you need to manually split the result of the expand into Lo and Hi here.  

https://github.com/llvm/llvm-project/pull/125207