type narrowing needs a conservative alias/escape analysis for correctness

[leanderBehr]

func main() {
  u: Long|Double = 0l

  !ur: Ref[Long|Double] = !u
  ur = 42.0

  print(u) # 4631107791820423168 Integer value of the representation of floating point value 42.0
  print_line()
  print(ur) # 42.000000 the floating point value
}

This example shows that the type narrowing is currently not save. We need some conservative analysis of references to avoid this.