[SLP][NFC]Improve perf of BoUpSLP::buildExternalUses function, NFC.

Perform required operations, only when the result is required + check if
value was not inserted already into the list of the externally used
scalars and early exit, if it was.

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -5514,6 +5514,7 @@ void BoUpSLP::reorderBottomToTop(bool IgnoreReorder) {
 
 void BoUpSLP::buildExternalUses(
     const ExtraValueToDebugLocsMap &ExternallyUsedValues) {
+  DenseMap<Value *, unsigned> ScalarToExtUses;
   // Collect the values that we need to extract from the tree.
   for (auto &TEPtr : VectorizableTree) {
     TreeEntry *Entry = TEPtr.get();
@@ -5527,13 +5528,18 @@ void BoUpSLP::buildExternalUses(
       Value *Scalar = Entry->Scalars[Lane];
       if (!isa<Instruction>(Scalar))
         continue;
-      int FoundLane = Entry->findLaneForValue(Scalar);
+      // All uses must be replaced already? No need to do it again.
+      auto It = ScalarToExtUses.find(Scalar);
+      if (It != ScalarToExtUses.end() && !ExternalUses[It->second].User)
+        continue;
 
       // Check if the scalar is externally used as an extra arg.
       const auto *ExtI = ExternallyUsedValues.find(Scalar);
       if (ExtI != ExternallyUsedValues.end()) {
+        int FoundLane = Entry->findLaneForValue(Scalar);
         LLVM_DEBUG(dbgs() << "SLP: Need to extract: Extra arg from lane "
-                          << Lane << " from " << *Scalar << ".\n");
+                          << FoundLane << " from " << *Scalar << ".\n");
+        ScalarToExtUses.try_emplace(Scalar, ExternalUses.size());
         ExternalUses.emplace_back(Scalar, nullptr, FoundLane);
       }
       for (User *U : Scalar->users()) {
@@ -5561,12 +5567,20 @@ void BoUpSLP::buildExternalUses(
             continue;
           }
           U = nullptr;
+          if (It != ScalarToExtUses.end()) {
+            ExternalUses[It->second].User = nullptr;
+            break;
+          }
         }
 
+        int FoundLane = Entry->findLaneForValue(Scalar);
         LLVM_DEBUG(dbgs() << "SLP: Need to extract:" << *UserInst
-                          << " from lane " << Lane << " from " << *Scalar
+                          << " from lane " << FoundLane << " from " << *Scalar
                           << ".\n");
+        It = ScalarToExtUses.try_emplace(Scalar, ExternalUses.size()).first;
         ExternalUses.emplace_back(Scalar, U, FoundLane);
+        if (!U)
+          break;
       }
     }
   }