[llvm] [VPlan] Run narrowInterleaveGroups during general VPlan optimizations. (PR #149706)

[David Sherwood via llvm-commits]

================
@@ -4011,24 +4031,44 @@ void VPlanTransforms::narrowInterleaveGroups(VPlan &Plan, ElementCount VF,
     auto *WideMember0 = dyn_cast_or_null<VPWidenRecipe>(
         InterleaveR->getStoredValues()[0]->getDefiningRecipe());
     if (!WideMember0)
-      return;
+      return nullptr;
     for (const auto &[I, V] : enumerate(InterleaveR->getStoredValues())) {
       auto *R = dyn_cast_or_null<VPWidenRecipe>(V->getDefiningRecipe());
       if (!R || R->getOpcode() != WideMember0->getOpcode() ||
           R->getNumOperands() > 2)
-        return;
+        return nullptr;
       if (any_of(enumerate(R->operands()),
                  [WideMember0, Idx = I](const auto &P) {
                    const auto &[OpIdx, OpV] = P;
                    return !canNarrowLoad(WideMember0, OpIdx, OpV, Idx);
                  }))
-        return;
+        return nullptr;
     }
     StoreGroups.push_back(InterleaveR);
   }
 
   if (StoreGroups.empty())
-    return;
+    return nullptr;
+
+  // All interleave groups in Plan can be narrowed for VFToOptimize. Split the
+  // original Plan into 2: a) a new clone which contains all VFs of Plan, except
+  // VFToOptimize, and b) the original Plan with VFToOptimize as single VF.
+  std::unique_ptr<VPlan> NewPlan;
+  if (size(Plan.vectorFactors()) != 1) {
+    NewPlan = std::unique_ptr<VPlan>(Plan.duplicate());
+    Plan.setVF(*VFToOptimize);
+    bool First = true;
+    for (ElementCount VF : NewPlan->vectorFactors()) {
+      if (VF == VFToOptimize)
+        continue;
+      if (First) {
+        NewPlan->setVF(VF);
+        First = false;
+        continue;
+      }
+      NewPlan->addVF(VF);
----------------
david-arm wrote:

This feels a bit cumbersome. It would be nice if `addVF` could be made to work without any existing VF, then you could rewrite the loop as:

```
  for (ElementCount VF : NewPlan->vectorFactors())
    if (VF != VFToOptimize)
      NewPlan->addVF(VF);
```

https://github.com/llvm/llvm-project/pull/149706