genemit: Add an internal queue

genemit: Add an internal queue

An earlier version of this series wanted to collect information
about all the gen_* functions that are going to be generated.
The current version no longer does that, but the queue seemed
worth keeping anyway, since it gives a more consistent structure.

gcc/
	* genemit.cc (queue): New static variable.
	(maybe_queue_insn): New function, split out from...
	(gen_insn): ...here.
	(queue_expand): New function, split out from...
	(gen_expand): ...here.
	(gen_split): New function, split out from...
	(queue_split): ...here.
	(main): Queue definitions for later processing rather than
	emitting them on the fly.