[llvm] [DAGCombine] Simplify partial_reduce_*mla with constant. (PR #138289)

[Benjamin Maxwell via llvm-commits]

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@@ -12612,47 +12612,63 @@ SDValue DAGCombiner::visitMHISTOGRAM(SDNode *N) {
   return SDValue();
 }
 
-// Makes PARTIAL_REDUCE_*MLA(Acc, MUL(ZEXT(LHSExtOp), ZEXT(RHSExtOp)),
-// Splat(1)) into
-// PARTIAL_REDUCE_UMLA(Acc, LHSExtOp, RHSExtOp).
-// Makes PARTIAL_REDUCE_*MLA(Acc, MUL(SEXT(LHSExtOp), SEXT(RHSExtOp)),
-// Splat(1)) into
-// PARTIAL_REDUCE_SMLA(Acc, LHSExtOp, RHSExtOp).
+// partial_reduce_*mla(acc, mul(ext(a), ext(b)))
+// -> partial_reduce_*mla(acc, a, b)
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MacDue wrote:

I think this is still missing the `splat(1)` RHS operand. 

https://github.com/llvm/llvm-project/pull/138289