makeReturn() {
    const forkContext = this.forkContext;

    if (forkContext.reachable) {
      getReturnContext(this).returnedForkContext.add(forkContext.head);
      forkContext.replaceHead(forkContext.makeUnreachable(-1, -1));
    }
  }
Makes a path for a `return` statement.

It registers the head segment to a context of `return`.
It makes new unreachable segment, then it set the head with the segment.
@returns {void}