Move bitmask routines to vrange base class.

Move bitmask routines to vrange base class.

Any range can theoretically have a bitmask of set bits.  This patch
moves the bitmask accessors to the base class.  This cleans up some
users in IPA*, and will provide a cleaner interface when prange is in
place.

gcc/ChangeLog:

	* ipa-cp.cc (propagate_bits_across_jump_function): Access bitmask
	through base class.
	(ipcp_store_vr_results): Same.
	* ipa-prop.cc (ipa_compute_jump_functions_for_edge): Same.
	(ipcp_get_parm_bits): Same.
	(ipcp_update_vr): Same.
	* range-op-mixed.h (update_known_bitmask): Change argument to vrange.
	* range-op.cc (update_known_bitmask): Same.
	* value-range.cc (vrange::update_bitmask):  New.
	(irange::set_nonzero_bits): Move to vrange class.
	(irange::get_nonzero_bits): Same.
	* value-range.h (class vrange): Add update_bitmask, get_bitmask,
	get_nonzero_bits, and set_nonzero_bits.
	(class irange): Make bitmask methods virtual overrides.
	(class Value_Range): Add get_bitmask and update_bitmask.