Stream out endpoints for frange.

We only stream out the FP properties for global float ranges
(currently only NAN).  The following patch adds the endpoints as well.

gcc/ChangeLog:

	* value-range-storage.cc (frange_storage_slot::set_frange): Save
	endpoints.
	(frange_storage_slot::get_frange): Restore endpoints.
	* value-range-storage.h (class frange_storage_slot): Add endpoint
	fields.