Accept a vrange in get_legacy_range.

Accept a vrange in get_legacy_range.

In preparation for prange, make get_legacy_range take a generic
vrange, not just an irange.

gcc/ChangeLog:

	* value-range.cc (get_legacy_range): Make static and add another
	version of get_legacy_range that takes a vrange.
	* value-range.h (class irange): Remove unnecessary friendship with
	get_legacy_range.
	(get_legacy_range): Accept a vrange.