FRANKFURT Siemens (SIEGn.DE) has agreed to buy Mentor Graphics (MENT.O) for $37.25 per share in cash, valuing the U.S. software group at $4.5 billion.Siemens said on Monday that it expects the deal to result in savings that would lift earnings before interest and tax (EBIT) by more than 100 million euros ($108 million) within four years.People familiar...