Replace the correct node when replacing in void sequences

Close #611.

test/compress/issue-611.js

@@ -0,0 +1,21 @@
+issue_611: {
+  options = {
+    sequences: true,
+    side_effects: true
+  };
+  input: {
+    define(function() {
+      function fn() {}
+      if (fn()) {
+        fn();
+        return void 0;
+      }
+    });
+  }
+  expect: {
+    define(function() {
+      function fn(){}
+      if (fn()) return void fn();
+    });
+  }
+}