general improvements around AST_ForIn (#2796)

- compress using `collapse_vars`
- remove unused `name`
- simplify `loop_body`

lib/parse.js

@@ -1054,12 +1054,10 @@ function parse($TEXT, options) {
     };
 
     function for_in(init) {
-        var lhs = init instanceof AST_Var ? init.definitions[0].name : null;
         var obj = expression(true);
         expect(")");
         return new AST_ForIn({
             init   : init,
-            name   : lhs,
             object : obj,
             body   : in_loop(statement)
         });