Keep x_range_query NULL for global ranges.

Keep x_range_query NULL for global ranges.

Instead of x_range_query always pointing to an object, have it default to
NULL and return a pointer to the global query in that case.

	* function.c (allocate_struct_function): Don't set x_range_query.
	* function.h (get_range_query): Move to value-query.h.
	* gimple-range.cc (enable_ranger): Check that query is currently NULL.
	(disable_ranger): Clear function current query field.
	* value-query.cc (get_global_range_query): Relocate to:
	* value-query.h (get_global_range_query): Here and inline.
	(get_range_query): Relocate here from function.h.