Mark vars in AST of ||, &&, if, and unless as generated

[arnodirlam]

Commit 19c628a introduced a private AST helper Kernel.x_is_false_or_nil/0 to add generated: true to a generated variable x in generated guards for of ||, &&, if, and unless (diff).

However, the variable x, on which the guards refer to, is not marked as generated: true, and thus has a different AST signature.

For reference, see this diff (expand to see lines 1070-1135).

This PR fixes this by adding another private AST helper Kernel.x_var/0 to add generated: true to the generated variables x as well.

[josevalim]

Thank you @arnodirlam. Can you please explain when/why this change would be necessary?