fix corner case in reduce_vars (#4203)

fixes #4198

lib/compress.js

@@ -6640,7 +6640,13 @@ merge(Compressor.prototype, {
                 if (def.scope === scope) return true;
                 return !scope.variables.has(node.name) && !scope.globals.has(node.name);
             }
-            return def.scope === scope || !scope.find_variable(node);
+            if (def.scope === scope) return true;
+            var s = def.scope;
+            do {
+                s = s.parent_scope;
+                if (s.variables.has(node.name)) return false;
+            } while (s !== scope);
+            return true;
         }) ? make_node(AST_Var, self, {
             definitions: self.definitions.map(function(defn) {
                 var name = make_node(AST_SymbolVar, defn.name, defn.name);