Make ssa_cache a range_query.

Make ssa_cache a range_query.

By providing range_of_expr as a range_query, we can fold and do other
interesting things using values from the global table.  Make ranger's
knonw globals available via const_query.

	* gimple-range-cache.cc (ssa_cache::range_of_expr): New.
	* gimple-range-cache.h (class ssa_cache): Inherit from range_query.
	(ranger_cache::const_query): New.
	* gimple-range.cc (gimple_ranger::const_query): New.
	* gimple-range.h (gimple_ranger::const_query): New prototype.