remove VerifyRegSubReg and cleanup region infer

We used to make region->region edges part of the verify set, but this
change stores them like other edges, as a full-fledged constraint.
Besides making the code somewhat cleaner, this allows them to be more
easily dropped as part of `pop_skolemized`. This change also refactors
the code a bit to remove some intermediate data structures that are no
longer particular useful (e.g., VarValue).

Author: nikomatsakis

src/librustc/infer/region_inference/graphviz.rs

@@ -213,8 +213,12 @@ fn constraint_to_nodes(c: &Constraint) -> (Node, Node) {
     match *c {
         Constraint::ConstrainVarSubVar(rv_1, rv_2) =>
             (Node::RegionVid(rv_1), Node::RegionVid(rv_2)),
-        Constraint::ConstrainRegSubVar(r_1, rv_2) => (Node::Region(r_1), Node::RegionVid(rv_2)),
-        Constraint::ConstrainVarSubReg(rv_1, r_2) => (Node::RegionVid(rv_1), Node::Region(r_2)),
+        Constraint::ConstrainRegSubVar(r_1, rv_2) =>
+            (Node::Region(r_1), Node::RegionVid(rv_2)),
+        Constraint::ConstrainVarSubReg(rv_1, r_2) =>
+            (Node::RegionVid(rv_1), Node::Region(r_2)),
+        Constraint::ConstrainRegSubReg(r_1, r_2) =>
+            (Node::Region(r_1), Node::Region(r_2)),
     }
 }