Modulo other issues/discussions, here is a version of this patch that implements CREATE OR REPLACE ROLE just by handing off to AlterRole if it's determined that the role already exists; presumably any/all additional considerations would need to be added in both places were there a separate code path for this.
It might be worth refactoring the AlterRole into a helper if there are any deviations in messages, etc, but could be a decent approach to handling the problem (which arguably would have similar restrictions/requirements in ALTER ROLE itself) in a single location.
Best,
David