Discard is not always treated as an explicit return and flow control can continue for required derivative calculations. This behaviour is different in Metal vs OpenGL. Adding return after discards ensures consistency in expectation as behaviour is well-defined.
Authored by Apple: Michael Parkin-White
Ref T96261
Reviewed By: fclem
Maniphest Tasks: T96261
Differential Revision: https://developer.blender.org/D17199