[Vectorize] Fix -Wunused-variable in DependencyGraph.h (NFC)

/llvm-project/llvm/include/llvm/Transforms/Vectorize/SandboxVectorizer/DependencyGraph.h:231:10:
error: unused variable 'Inserted' [-Werror,-Wunused-variable]
    auto Inserted = MemPreds.insert(PredN).second;
         ^
1 error generated.

Author: DamonFool

llvm/include/llvm/Transforms/Vectorize/SandboxVectorizer/DependencyGraph.h

@@ -228,7 +228,7 @@ class MemDGNode final : public DGNode {
   /// UnscheduledSuccs counter of the predecessor if this node has not been
   /// scheduled.
   void addMemPred(MemDGNode *PredN) {
-    auto Inserted = MemPreds.insert(PredN).second;
+    [[maybe_unused]] auto Inserted = MemPreds.insert(PredN).second;
     assert(Inserted && "PredN already exists!");
     if (!Scheduled) {
       ++PredN->UnscheduledSuccs;