`return`-reachability analysis should take unconditional `throw` into account

[anton-trunov]

For instance, something like this should not result in an error message from the analysis:

fun test1(): Int {
  throw(200);
}

or, a bit more realistic example:

fun test2(c: Bool): Int {
  if (c) {
    throw(200);
  } else {
    return 42;
  }
}