[RFC] Introduce convergence control intrinsics

Thanks! I am sure we can wait two more weeks with the hope that things will move forward. FWIW, the original RFC in D85603 had received a lot of discussion. This new RFC only clarifies and simplifies some things, so most of the original discussion is still relevant. The impression created back then was that people understood what is being introduced here and why, but that RFC never reached a conclusion.