JavaScript: Add utility predicate getBasePortal(i).

This iterates the existing `getBasePortal()` predicate `i` times.

javascript/ql/src/semmle/javascript/dataflow/Portals.qll

@@ -96,6 +96,20 @@ class Portal extends TPortal {
   cached
   ReturnPortal getReturn() { result.getBasePortal() = this }
 
+  /**
+   * Gets the `i`th base portal of this portal.
+   *
+   * The `0`th base portal is the portal itself, the `n+1`st base portal is the `n`th base portal
+   * of the portal `p` of which this is a member, instance, parameter, or return portal.
+   */
+  cached
+  Portal getBasePortal(int i) {
+    i = 0 and
+    result = this
+    or
+    result = this.(CompoundPortal).getBasePortal().getBasePortal(i - 1)
+  }
+
   /**
    * Gets a textual representation of this portal.
    *