Add a signal handler for SIGINT to ir_minimizer_main that prints out current IR

When `ir_minimizer_main` is being run manually, sometimes you'll get a long way into a run and realize that the flags were wrong somehow. It'd be nice to provide the capability to pick up from where you left off without spamming the console with the IR every time. One way of doing this is to install a signal handler that will detect SIGINT and print out the last failing IR.