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

[Sander de Smalen via llvm-commits]

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@@ -1607,6 +1607,13 @@ class SelectionDAG {
   /// the target's desired shift amount type.
   SDValue getShiftAmountOperand(EVT LHSTy, SDValue Op);
 
+  // Expands PARTIAL_REDUCE_S/UMLA nodes
----------------
sdesmalen-arm wrote:

```suggestion
  // Expands PARTIAL_REDUCE_S/UMLA nodes to a series of simpler operations,
  // consisting of zext/sext, extract_subvector, mul and add operations.
```

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