Refactor RPO VN API to allow timevar tracking

Refactor RPO VN API to allow timevar tracking

The following refactors things sligtly so "utility" use of the RPO VN
machinery gets its own timevar when invoked from other passes.

	* timevar.def (TV_TREE_RPO_VN): New.
	* tree-ssa-sccvn.h (do_rpo_vn): Remove one overload.
	* tree-ssa-sccvn.cc (do_rpo_vn_1): Rename the worker.
	(do_rpo_vn): Unify the public API, track with TV_TREE_RPO_VN.
	(pass_fre::execute): Adjust.
	* tree-ssa-uninit.cc (execute_early_warn_uninitialized): Adjust.