I see no upside to telling them about your plans. The company will need fewer people, but if you're in a key role (for example, rare expertise), you might not be offered severance or early retirement. Restructuring efforts following M&As sometimes benefit people like you who are a bit too young for regular retirement. These folks may enjoy "sweeteners" to get them to retire early -- enhanced retirement options, for example, that "bridge" them to retirement age. That's the best case scenario for you.
Even if your employer does not offer an early retirement package, the severance terms may be negotiable, particularly if you are close to retirement, a long-term employee, and retain good counsel. If it were me, I'd make the company believe I plan to work there forever and haven't thought about retirement at all. Make them pay you to go away. I also would not confide my true plans in coworkers, my boss, or anyone else.