fix accounting after conversion to assignment (#2847)

Missing reference to `AST_SymbolRef` created by `unused` causes `collapse_vars` to misbehave.

fixes #2846

lib/compress.js

@@ -3038,9 +3038,11 @@ merge(Compressor.prototype, {
                                 if (var_defs.length > 1 && (!def.value || sym.orig.indexOf(def.name) > sym.eliminated)) {
                                     compressor.warn("Dropping duplicated definition of variable {name} [{file}:{line},{col}]", template(def.name));
                                     if (def.value) {
+                                        var ref = make_node(AST_SymbolRef, def.name, def.name);
+                                        sym.references.push(ref);
                                         var assign = make_node(AST_Assign, def, {
                                             operator: "=",
-                                            left: make_node(AST_SymbolRef, def.name, def.name),
+                                            left: ref,
                                             right: def.value
                                         });
                                         if (fixed_ids[sym.id] === def) {