Rollup merge of #123980 - WaffleLapkin:graph-average-refactor, r=wesleywiser

Add an opt-in to store incoming edges in `VecGraph` + misc

r? ```@fmease```

needed for #123939

compiler/rustc_infer/src/infer/region_constraints/leak_check.rs

@@ -382,7 +382,7 @@ impl<'tcx> MiniGraph<'tcx> {
                 edges.push((source_node, target_node));
             },
         );
-        let graph = VecGraph::new(nodes.len(), edges);
+        let graph = VecGraph::<_, false>::new(nodes.len(), edges);
         let sccs = Sccs::new(&graph);
         Self { nodes, sccs }
     }