Avoid storing simple expressions like -x in temporaries

[efremale]

Example:

double f(double x) {
  double t = 2 * (-x);
  return t;
}

->

double f_darg0(double x) {
    double _t0 = -x;
    double _d_t = 0 * _t0 + 2 * -1.;
    double t = 2 * _t0;
    return _d_t;
}

In this case, double _t0 = -x seems useless so it makes sense to add such simple unary operators to a list of cases for which temporaries are not generated (now it is literals and references). On the other hand, results of unary operators like ++x must be stored to avoid repeated increment.

[efremale]

Fixed in #129