Refactor checks for nosideeffects extern calls to omit the Function type

The problem with 9c4a09e1b3bfecf3eb779d9cc75e1dfc3987d8f4 was that normal functions have the type `Function` which is defined in the [es3.js](https://github.com/google/closure-compiler/blob/master/externs/es3.js#L361) extern.
Can you merge all these cases to sth like this:

if (n.isFunction()) {
  // use these two. The name should be non null
  NodeUtil.getFunctionName(n)
  NodeUtil.getBestJsdocInfo(n)
}