[llvm] [VPlan] First step towards VPlan cost modeling. (PR #67934)

[via llvm-commits]

================
@@ -973,6 +973,93 @@ void VPWidenRecipe::execute(VPTransformState &State) {
 #endif
 }
 
+InstructionCost VPWidenRecipe::computeCost(ElementCount VF,
+                                           VPCostContext &Ctx) {
+  VPWidenRecipe *Cur = this;
+  // Check if the recipe is used in a reduction chain. Let the legacy cost-model
+  // handle that case for now.
+  while (Cur->getNumUsers() == 1) {
+    if (auto *Next = dyn_cast<VPWidenRecipe>(*Cur->user_begin())) {
+      Cur = Next;
+      continue;
+    }
+    if (isa<VPReductionRecipe>(*Cur->user_begin()))
+      return InstructionCost::getInvalid();
+    break;
----------------
ayalz wrote:

```suggestion
    VPUser *Next = *Cur->user_begin();
    if (isa<VPReductionRecipe>(Next))
      return InstructionCost::getInvalid();
    if (!isa<VPWidenRecipe>(Next))
      break;
   Cur = cast<VPWidenRecipe>(Next);
```

https://github.com/llvm/llvm-project/pull/67934