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.
Well that validates my approach then. I'll address the review issues and roll these checks into the CheckSideEffects pass.