Fix hoisting the var in ForIn

Close #913

test/compress/issue-913.js

@@ -0,0 +1,20 @@
+keep_var_for_in: {
+    options = {
+        hoist_vars: true,
+        unused: true
+    };
+    input: {
+        (function(obj){
+            var foo = 5;
+            for (var i in obj)
+                return foo;
+        })();
+    }
+    expect: {
+        (function(obj){
+            var i, foo = 5;
+            for (i in obj)
+                return foo;
+        })();
+    }
+}