Provide an API for ipa_vr.

Provide an API for ipa_vr.

This patch encapsulates the ipa_vr internals into an API.  It also
makes it type agnostic, in preparation for upcoming changes to IPA.

Interestingly, there's a 0.44% improvement to IPA-cp, which I'm sure
we'll soak up with future changes in this area :).

gcc/ChangeLog:

	* ipa-cp.cc (ipa_value_range_from_jfunc): Use new ipa_vr API.
	(ipcp_store_vr_results): Same.
	* ipa-prop.cc (ipa_vr::ipa_vr): New.
	(ipa_vr::get_vrange): New.
	(ipa_vr::set_unknown): New.
	(ipa_vr::streamer_read): New.
	(ipa_vr::streamer_write): New.
	(write_ipcp_transformation_info): Use new ipa_vr API.
	(read_ipcp_transformation_info): Same.
	(ipa_vr::nonzero_p): Delete.
	(ipcp_update_vr): Use new ipa_vr API.
	* ipa-prop.h (class ipa_vr): Provide an API and hide internals.
	* ipa-sra.cc (zap_useless_ipcp_results): Use new ipa_vr API.

gcc/testsuite/ChangeLog:

	* gcc.dg/ipa/pr78121.c: Adjust for vrange::dump use.
	* gcc.dg/ipa/vrp1.c: Same.
	* gcc.dg/ipa/vrp2.c: Same.
	* gcc.dg/ipa/vrp3.c: Same.
	* gcc.dg/ipa/vrp4.c: Same.
	* gcc.dg/ipa/vrp5.c: Same.
	* gcc.dg/ipa/vrp6.c: Same.
	* gcc.dg/ipa/vrp7.c: Same.
	* gcc.dg/ipa/vrp8.c: Same.