Check for undefined before not returning a constant value

Check for undefined before not returning a constant value

Don't return UNDEFINED for a range in an unreachable block if the global
value evaluates to a constant.  Return that constant instead.

	PR tree-optimization/97515
	* value-query.cc (range_query::value_of_expr): If the result is
	UNDEFINED, check to see if the global value is a constant.
	(range_query::value_on_edge): Ditto.