Add storage support for prange.

Add storage support for prange.

gcc/ChangeLog:

	* value-range-storage.cc (vrange_allocator::clone_varying): Add
	prange support.
	(vrange_allocator::clone_undefined): Same.
	(vrange_storage::alloc): Same.
	(vrange_storage::set_vrange): Same.
	(vrange_storage::get_vrange): Same.
	(vrange_storage::fits_p): Same.
	(vrange_storage::equal_p): Same.
	(prange_storage::alloc): New.
	(prange_storage::prange_storage): New.
	(prange_storage::set_prange): New.
	(prange_storage::get_prange): New.
	(prange_storage::equal_p): New.
	(prange_storage::fits_p): New.
	* value-range-storage.h (class prange_storage): Add prange support.