Start node has no immediate dominator

Change the immediate_dominator return type to Option, and use None to
indicate that node has no immediate dominator.

Also fix the issue where the start node would be returned as its own
immediate dominator.

compiler/rustc_const_eval/src/transform/validate.rs

@@ -164,7 +164,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                 if let Some(root) = post_contract_node.get(&bb) {
                     break *root;
                 }
-                let parent = doms.immediate_dominator(bb);
+                let parent = doms.immediate_dominator(bb).unwrap();
                 dom_path.push(bb);
                 if !self.body.basic_blocks[parent].is_cleanup {
                     break bb;