// checkgoto checks that a goto from from to to does not
// jump into a block or jump over variable declarations.
// It is a copy of checkgoto in the pre-SSA backend,
// modified only for line number handling.
// TODO: document how this works and why it is designed the way it is.
func (s *state) checkgoto(from *Node, to *Node) {
    if from.Sym == to.Sym {
        return
    }
...