[llvm] [LV] Reduce register usage for scaled reductions (PR #133090)

[Sander de Smalen via llvm-commits]

================
@@ -4874,6 +4874,20 @@ void LoopVectorizationCostModel::collectElementTypesForWidening() {
   }
 }
 
+/// Get the VF scaling factor applied to the recipe's output, if the recipe has
+/// one.
+static unsigned getVFScaleFactor(VPRecipeBase *R) {
+  if (isa<VPPartialReductionRecipe, VPReductionPHIRecipe>(R)) {
+    auto *ReductionR = dyn_cast<VPReductionPHIRecipe>(R);
+    auto *PartialReductionR =
+        ReductionR ? nullptr : dyn_cast<VPPartialReductionRecipe>(R);
+    unsigned ScaleFactor = ReductionR ? ReductionR->getVFScaleFactor()
+                                      : PartialReductionR->getVFScaleFactor();
+    return ScaleFactor;
+  }
----------------
sdesmalen-arm wrote:

Am I missing something, or can this just be:
```suggestion
  if (auto *RR = dyn_cast<VPReductionPHIRecipe>(R))
    return RR->getVFScaleFactor();
  if (auto *RR =dyn_cast<VPPartialReductionRecipe>(R))
    return RR->getVFScaleFactor();
```
?

https://github.com/llvm/llvm-project/pull/133090