Infer types across conditional branches for same variables

[josevalim]

Following up on #15459, we need to make it so Kernel.or, Kernel.and, Kernel.|| and Kernel.&& can all pass this test:

            assert typecheck!(
              [x],
              cond do
                is_integer(x) or is_boolean(x) -> {:one, x}
                true -> {:two, x}
              end
            ) == opt_union(
              tuple([atom([:one]), opt_union(integer(), boolean())]),
              tuple([atom([:two]), negation(opt_union(integer(), boolean()))])
           )

As far as I know, none of them do it today. And it is tricky because is_integer(x) or is_boolean(y) gets a different result. We already have this machinery in guards, but we need to repurpose it.

Originally posted by @josevalim in #15450