Return a bool result for union, and add performance improvements.

Return a bool result for union, and add performance improvements.

Union_ returns a boolean indicating if the operation changes the range.
Also optimize the common single-pair UNION single-pair case.

	* gimple-range-edge.cc (calc_switch_ranges): Check union return value.
	* value-range.cc (irange::legacy_verbose_union_): Add return value.
	(irange::irange_single_pair_union): New.
	(irange::irange_union): Add return value.
	* value-range.h (class irange): Adjust prototypes.