Return TRUE only when a global value is updated.

Return TRUE only when a global value is updated.

set_range_info should return TRUE only when it sets a new value.  VRP no
longer overwrites global ranges DOM has set.  Check for ranges in the
final listing.

	gcc/
	* tree-ssanames.cc (set_range_info): Return true only if the
	current value changes.

	gcc/testsuite/
	* gcc.dg/pr93917.c: Check for ranges in final optimized listing.
	* gcc.dg/tree-ssa/vrp-unreachable.c: Ditto.