Remove unused return is not recursive

[eponier]

If removing a return reveals a new opportunity to remove a return, the compiler does not take advantage of it.

For instance,

fn g () -> reg u32 {
  reg u32 x;
  x = 0;
  return x;
}

fn f () -> reg u32 {
  reg u32 x;
  x = g ();
  return x;
}

export fn main () -> reg u32 {
  reg u32 res;
  res = 0;
  _ = f();
  return res;
}

The compiler removes the return value of f and thus could remove the return value of g, but it does not do it.