[VPlan] Match legacy behavior w.r.t. using pointer phis as scalar addrs.

When the legacy cost model scalarizes loads that are used as addresses
for other loads and stores, it looks to phi nodes, if they are direct
address operands of loads/stores. Match this behavior in
isUsedByLoadStoreAddress, to fix a divergence between legacy and
VPlan-based cost model.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -3162,7 +3162,17 @@ static bool isUsedByLoadStoreAddress(const VPUser *V) {
 
   while (!WorkList.empty()) {
     auto *Cur = dyn_cast<VPSingleDefRecipe>(WorkList.pop_back_val());
-    if (!Cur || !Seen.insert(Cur).second || isa<VPBlendRecipe>(Cur))
+    if (!Cur || !Seen.insert(Cur).second)
+      continue;
+
+    auto *Blend = dyn_cast<VPBlendRecipe>(Cur);
+    // Skip blends that use V only through a compare by checking if any incoming
+    // value was already visited.
+    if (Blend && none_of(seq<unsigned>(0, Blend->getNumIncomingValues()),
+                         [&](unsigned I) {
+                           return Seen.contains(
+                               Blend->getIncomingValue(I)->getDefiningRecipe());
+                         }))
       continue;
 
     for (VPUser *U : Cur->users()) {
@@ -3183,7 +3193,13 @@ static bool isUsedByLoadStoreAddress(const VPUser *V) {
       }
     }
 
-    append_range(WorkList, cast<VPSingleDefRecipe>(Cur)->users());
+    // The legacy cost model only supports scalarization loads/stores with phi
+    // addresses, if the phi is directly used as load/store address. Don't
+    // traverse further for Blends.
+    if (Blend)
+      continue;
+
+    append_range(WorkList, Cur->users());
   }
   return false;
 }