Accept pointer instance types on falsey conditional branches

[HertzDevil]

Crystal currently only allows Bool and Nil when performing flow typing on a falsey variable, but all Pointer instances can also be falsey. This leads to the following bug:

x = false || Pointer(Void).null if true # => Pointer(Void).null
typeof(x)                               # => (Bool | Pointer(Void) | Nil)

if !x
  typeof(x) # => (Bool | Nil)
  p x       # Invalid memory access (signal 11) at address 0xb4
end

This PR adds all Pointer instance types to those falsey branches, so that the inner typeof will report the same type as the outer one.

[sdogruyol]

Thank you @HertzDevil 🙏