Lazy dominator tree construction in borrowck

Motivated by the observation that sometimes constructed dominator tree
was never queried.

compiler/rustc_data_structures/src/graph/dominators/mod.rs

@@ -268,10 +268,6 @@ pub struct Dominators<N: Idx> {
 }
 
 impl<Node: Idx> Dominators<Node> {
-    pub fn dummy() -> Self {
-        Self { post_order_rank: IndexVec::new(), immediate_dominators: IndexVec::new() }
-    }
-
     pub fn is_reachable(&self, node: Node) -> bool {
         self.immediate_dominators[node].is_some()
     }