[llvm] [DAGCombiner] Add generic DAG combine for ISD::PARTIAL_REDUCE_MLA (PR #127083)

[Benjamin Maxwell via llvm-commits]

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@@ -12497,6 +12501,50 @@ SDValue DAGCombiner::visitMHISTOGRAM(SDNode *N) {
   return SDValue();
 }
 
+SDValue DAGCombiner::visitPARTIAL_REDUCE_MLA(SDNode *N) {
+  // Makes PARTIAL_REDUCE_MLA(Acc, MUL(EXT(MulOpLHS), EXT(MulOpRHS)), Splat(1))
+  // into PARTIAL_REDUCE_MLA(Acc, MulOpLHS, MulOpRHS)
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MacDue wrote:

I'm slightly inclined towards keeping the combine as one function right now. Since, when it's folding the extends and the `mul` at the same time, then there's no issue since the extends the partial reduce expansion will introduce already existed in the input. Folding the `mul` on its own is implicitly adding extends that did not exist previously (if the `mul` you fold into the partial reduce does not have the same element type as the partial reduce accumulator, which is likely). 

https://github.com/llvm/llvm-project/pull/127083