perform reduce_vars on safe literals (#2351)

- constant expression
- single reference
- same scope
- not across loop body

test/compress/drop-unused.js

@@ -751,12 +751,12 @@ issue_1583: {
     expect: {
         function m(t) {
             (function(e) {
-                t = e();
-            })(function() {
-                return (function(a) {
-                    return a;
-                })(function(a) {});
-            });
+                t = function() {
+                    return (function(a) {
+                        return function(a) {};
+                    })();
+                }();
+            })();
         }
     }
 }