Don't add irrelevant items to queue in DwarfCompileUnit::createScopeChildrenDIE (NFC)

[aaronpuchert]

Summary

Instead of popping them and then immediately throwing them away, we can
just filter out globals and items in different scopes before adding them
to WorkList. Shouldn't change anything but keep the queue smaller.

[aaronpuchert]

Also use the non-modifying DenseMapBase::lookup instead of operator[].

[aprantl]

Seems reasonable.

llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp

sortLocalVars(SmallVectorImpl<DbgVariable *> &Input) {
 
   // Perform a stable topological sort by doing a DFS.
   while (!WorkList.empty()) {
     auto Item = WorkList.back();
     DbgVariable *Var = Item.getPointer();
     bool visitedAllDependencies = Item.getInt();
     WorkList.pop_back();
 
-    // Dependency is in a different lexical scope or a global.
-    if (!Var)
-      continue;
+    assert(Var);
 
     // Already handled.
     if (Visited.count(Var))
       continue;
 
     // Add to Result if all dependencies are visited.
     if (visitedAllDependencies) {
       Visited.insert(Var);
       Result.push_back(Var);
       continue;
     }
 
     // Detect cycles.
     auto Res = Visiting.insert(Var);
     if (!Res.second) {
       assert(false && "dependency cycle in local variables");
       return Result;
     }
 
     // Push dependencies and this node onto the worklist, so that this node is
     // visited again after all of its dependencies are handled.
     WorkList.push_back({Var, 1});
     for (auto *Dependency : dependencies(Var)) {
-      auto Dep = dyn_cast_or_null<const DILocalVariable>(Dependency);
-      WorkList.push_back({DbgVar[Dep], 0});
+      // Don't add dependency if it is in a different lexical scope or a global.
+      if (const auto *Dep = dyn_cast<const DILocalVariable>(Dependency))
+        if (DbgVariable *Var = DbgVar[Dep])
+          WorkList.push_back({Var, 0});
     }
   }
   return Result;
 }
 
 DIE *DwarfCompileUnit::createScopeChildrenDIE(LexicalScope *Scope,
                                               SmallVectorImpl<DIE *> &Children,
                                               bool *HasNonScopeChildren) {