fix corner case with top-level singular reference

lib/compress.js

@@ -711,7 +711,8 @@ merge(Compressor.prototype, {
                                 return make_node(AST_UnaryPrefix, candidate, candidate);
                             }
                             if (candidate instanceof AST_VarDef) {
-                                if (candidate.name.definition().references.length == 1) {
+                                var def = candidate.name.definition();
+                                if (def.references.length == 1 && (!def.global || compressor.toplevel(def))) {
                                     return maintain_this_binding(parent, node, candidate.value);
                                 }
                                 return make_node(AST_Assign, candidate, {
@@ -779,9 +780,7 @@ merge(Compressor.prototype, {
                     expr.expressions.forEach(extract_candidates);
                 } else if (expr instanceof AST_Definitions) {
                     expr.definitions.forEach(function(var_def) {
-                        if (var_def.value) {
-                            candidates.push(var_def);
-                        }
+                        if (var_def.value) candidates.push(var_def);
                     });
                 } else if (expr instanceof AST_SimpleStatement) {
                     extract_candidates(expr.body);

test/compress/collapse_vars.js

@@ -2059,3 +2059,21 @@ double_def: {
         (a = a && y)();
     }
 }
+
+toplevel_single_reference: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        var a;
+        for (var b in x) {
+            var a = b;
+            b(a);
+        }
+    }
+    expect: {
+        var a;
+        for (var b in x)
+            b(a = b);
+    }
+}