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

[Benjamin Maxwell via llvm-commits]

================
@@ -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(zext(a), zext(b)))
+// -> partial_reduce_umla(acc, a, b)
----------------
MacDue wrote:

```suggestion
// partial_reduce_*mla(acc, mul(ext(a), ext(b)), splat(1))
// -> partial_reduce_*mla(acc, a, b)
```
?

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