Replace unless/2 usages in the codebase

[sabiwara]

Extracted from the implementation of the :migrate option itself (#13769), for a leaner diff and simpler discussion.

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Thinking aloud: in the same way we're compiling !!x as a single case, we could do the same thing for if !x and if not x, making it equivalent to unless (or a more strict version of it in the case of not).

So instead of

case (case x do
        var2 when var2 === false or var2 === nil -> true
        _ -> false
      end) do
  false -> nil
  true -> 0
end

if !x, do: 0 would generate

case x do
  var2 when var2 === false or var2 === nil -> 0
  _ -> nil
end

Thoughts? I can send a follow-up PR for this.