Cleanup get_range_info

Cleanup get_range_info

Now that there is only one user of get_range_info() we can clean this up
to always return a range instead of pairs of wide_ints.

A follow-up patch will inline both get_range_info and get_ptr_nonnull
into its only remaining user.

gcc/ChangeLog:

	* tree-ssanames.c (get_range_info): Merge both copies of
	get_range_info into one that works with irange.
	* tree-ssanames.h (get_range_info): Remove version that works on
	wide_ints.