[asl] Prevent use of empty slices on integers and bitvectors.

[HadrienRenaud]

This makes it impossible to call a bracket-less getter with brackets.

See the now forbidden tests:

getter f1 => bits(4)
begin
  return '0000';
end

setter f1 = v: bits(4)
begin
  pass;
end

func main () => integer
begin
  // both of the following are forbidden
  let - = f1[];
  f1[] = '';
  
  return 0;
end