(b) A person domiciled in this State at the time such trust, or portion of a trust, became irrevocable, if it was revocable when such property was transferred to the trust but has subsequently become irrevocable.
For the purposes of the foregoing, a trust or portion of a trust is revocable if it is subject to a power, exercisable immediately or at any future time, to revest title in the person whose property constitutes such trust or portion of a trust, and a trust or portion of a trust becomes irrevocable when the possibility that such power may be exercised has been terminated.