Tweak merge_range API.

merge_range use to return TRUE if there was already a range.  Now it
returns TRUE if a new range is added, OR updates and existing range
with a new value.  FALSE is returned when the range already matches.

	* gimple-range-cache.cc (ssa_cache::merge_range): Change meaning
	of the return value.
	(ssa_cache::dump): Don't print GLOBAL RANGE header.
	(ssa_lazy_cache::merge_range): Adjust return value meaning.
	(ranger_cache::dump): Print GLOBAL RANGE header.