It is a really silly plan because the opamp is probably not designed to supply much more than 25 to 50 mA. If you want some idea of the wasted heat, take 5 from 12 and multiply by the load current. At 50 mA that would be about 350 mW of wasted heat. Depending on the package the opamp will sit there and cook. It might also go unstable and break into oscillation, or just sag down to some unpredictable level. You would be so much better off with an LDO regulator. Why in the name of ... are people so averse to using 3-terminal regulators? There just is no excuse or justification for all the nonsensical methods out there for regulating voltage without using a part designed for that purpose. Do we imagine that we are legends in our own minds and have just discovered the freaking holy grail?