Add gimple_range_type for statements.

Add gimple_range_type for statements.

The existing mechanisms for picking up the type of a statement are
inconsistent with the needs of ranger. Encapsulate all the bits
required to pick up the return type of a statement in one place, and check
whether the type is supported.

	* gimple-range-fold.cc (adjust_pointer_diff_expr): Use
	gimple_range_type.
	(fold_using_range::fold_stmt): Ditto.
	(fold_using_range::range_of_range_op): Ditto.
	(fold_using_range::range_of_phi): Ditto.
	(fold_using_range::range_of_call): Ditto.
	(fold_using_range::range_of_builtin_ubsan_call): Ditto.
	(fold_using_range::range_of_builtin_call): Ditto.
	(fold_using_range::range_of_cond_expr): Ditto.
	* gimple-range-fold.h (gimple_range_type): New.