I personally don't see the point of doing this. The "Axis of Evil" rhetoric blew up in the US face about a decade ago.
The US was aware that a variety of foreign entities and governments, to include the Saudis, give a variety of help and support to some terrorist organizations. But they have not declared them as terrorist organizations.
That entity, the IRGC, has official status within the Iranian polity and is an arm of Iranian government and policy - regardless of how they are or are not a group of people who make the US policy aims difficult in that region.
That doesn't make them terrorists, nor a terrorist organization.
They are one of host of problems to deal with.