[SLP] Rework `findBuildAggregate()` from ercursive form to iterative, NFC.

[mkuper]

LGTM with minor comments.

lib/Transforms/Vectorize/SLPVectorizer.cpp

4617

Any reason this isn't nullptr? It looks like you never use the initial value.

4628

Does the order really matter?
Anyway, if you think it does, I'm fine with this to keep it NFC.

lib/Transforms/Vectorize/SLPVectorizer.cpp

 }
 
 /// \brief Like findBuildVector, but looks backwards for construction of aggregate.
 ///
 /// \return true if it matches.
 static bool findBuildAggregate(InsertValueInst *IV,
                                SmallVectorImpl<Value *> &BuildVector,
                                SmallVectorImpl<Value *> &BuildVectorOpds) {
-  if (!IV->hasOneUse())
-    return false;
-  Value *V = IV->getAggregateOperand();
-  if (!isa<UndefValue>(V)) {
-    InsertValueInst *I = dyn_cast<InsertValueInst>(V);
-    if (!I || !findBuildAggregate(I, BuildVector, BuildVectorOpds))
-      return false;
-  }
+  Value *V = IV;
+  do {
     BuildVector.push_back(IV);
     BuildVectorOpds.push_back(IV->getInsertedValueOperand());
+    V = IV->getAggregateOperand();
+    if (isa<UndefValue>(V))
+      break;
+    IV = dyn_cast<InsertValueInst>(V);
+    if (!IV || !IV->hasOneUse())
+      return false;
+  } while (true);
+  std::reverse(BuildVector.begin(), BuildVector.end());
+  std::reverse(BuildVectorOpds.begin(), BuildVectorOpds.end());
   return true;
 }
 
 static bool PhiTypeSorterFunc(Value *V, Value *V2) {
   return V->getType() < V2->getType();
 }
 
 /// \brief Try and get a reduction value from a phi node.