refactor: switch ancestory/descendant logic [skip ci]

- Ref https://github.com/babel/babel/pull/4836#discussion_r88118790 @jridgewell

packages/babel-traverse/src/path/ancestry.js

@@ -179,14 +179,14 @@ export function getAncestry() {
  * A helper to find if `this` path is an ancestor of @param maybeDescendant
  */
 export function isAncestor(maybeDescendant) {
-  return !!maybeDescendant.findParent((parent) => parent === this);
+  return maybeDescendant.isDescendant(this);
 }
 
 /**
  * A helper to find if `this` path is a descendant of @param maybeAncestor
  */
 export function isDescendant(maybeAncestor) {
-  return maybeAncestor.isAncestor(this);
+  return !!this.findParent((parent) => parent === maybeAncestor);
 }
 
 export function inType() {