[SLP][NFC]Check an external user before trying to address it in debug dump, NFC

Author: alexey-bataev

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -15097,7 +15097,8 @@ InstructionCost BoUpSLP::getTreeCost(ArrayRef<Value *> VectorizedVals,
   for (ExternalUser &EU : ExternalUses) {
     LLVM_DEBUG(dbgs() << "SLP: Computing cost for external use of TreeEntry "
                       << EU.E.Idx << " in lane " << EU.Lane << "\n");
-    LLVM_DEBUG(dbgs() << "  User:" << *EU.User << "\n");
+    LLVM_DEBUG(if (EU.User) dbgs() << "  User:" << *EU.User << "\n";
+               else dbgs() << "  User: nullptr\n");
     LLVM_DEBUG(dbgs() << "  Use: " << EU.Scalar->getNameOrAsOperand() << "\n");
 
     // Uses by ephemeral values are free (because the ephemeral value will be