util: optimize promise introspection

Use V8's builtin ObjectIsPromise() to check that the value is a promise
before creating the promise mirror.  Reduces garbage collector strain
in the (common) non-promise case, which is beneficial when inspecting
deep object graphs.

PR-URL: https://github.com/nodejs/node/pull/3130


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>