Clean up portable op_topk (#7543)

The comparison function was repeated several times. Factor it out.

Differential Revision: [D67875394](https://our.internmc.facebook.com/intern/diff/D67875394/)

ghstack-source-id: 260406714
Pull Request resolved: https://github.com/pytorch/executorch/pull/7528

Co-authored-by: Scott Wolchok <swolchok@fb.com>