Omit return statements that don't have an argument or return undefined

[Rich-Harris]

This already happens in simple cases:

// input
function foo () {
  console.log(1);
  return;
  console.log(2);
}

// output
function foo(){console.log(1)}

But it fails if there's a declaration after the return, even an unused one:

// input
function foo () {
  console.log(1);
  return;
  console.log(2);

  function x () {}
}

// output
function foo(){console.log(1);return}