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

[Sander de Smalen via llvm-commits]

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@@ -135,6 +135,10 @@ static cl::opt<unsigned> SwitchPeelThreshold(
              "switch statement. A value greater than 100 will void this "
              "optimization"));
 
+static cl::opt<bool> NewPartialReduceLowering(
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sdesmalen-arm wrote:

nit: maybe add a FIXME to say that this flag is temporary, to help transition to do proper lowering with the new selectiondag nodes?

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