Fix handling of symbolic boolean comparisons

[m-carrasco]

Hi,

This PR completes the support for boolean symbolic comparisons. The previous code did not handle cases where the two operands are symbolic.

This test case can reproduce the issue:

import tools.aqua.concolic.Verifier;

public class Main {
    public static void main(String[] args)
    {
        foo(Verifier.nondetBoolean(), Verifier.nondetBoolean());

        System.exit(0);
    }

    public static boolean foo(boolean arg1, boolean arg2)
    {
      return arg1 == arg2;
    }
}

Best regards,
Manuel