fix corner case in reduce_vars (#3378)

fixes #3377

lib/compress.js

@@ -744,7 +744,7 @@ merge(Compressor.prototype, {
                 d.fixed = false;
             } else if (d.fixed) {
                 value = this.fixed_value();
-                if (value instanceof AST_Lambda && recursive_ref(tw, d)) {
+                if (recursive_ref(tw, d)) {
                     d.recursive_refs++;
                 } else if (value && ref_once(tw, compressor, d)) {
                     d.single_use = value instanceof AST_Lambda && !value.pinned()
@@ -2603,9 +2603,9 @@ merge(Compressor.prototype, {
     }
 
     function convert_to_predicate(obj) {
-        for (var key in obj) {
+        Object.keys(obj).forEach(function(key) {
             obj[key] = makePredicate(obj[key]);
-        }
+        });
     }
 
     var object_fns = [