Fix unsafe optimizations where guard tests could be removed

A repeated test could be optimized away. Example:

    bar(A) ->
        if is_bitstring(A) ->
                if is_binary(A) ->
                        binary;
                   true ->
                        bitstring
                end;
           true ->
                other
        end.

In the example, the `is_binary/1` test would be optimized away,
basically turning the example into:

    bar(A) ->
        if is_bitstring(A) ->
                bitstring;
           true ->
                other
        end.

Thanks user Marcus Kruse in the Elixir forum for noticing this bug.

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