So, I've looked into this again out of curiosity, because I do have an LLVM
patch that can be used to work around this. However, the patch introduces some
inefficiencies, and this is a game bug: it is clear from playing with the
provided apitrace that a derivative is taken after a non-uniform discard/kill,
which is undefined behaviour in GLSL.

Does anybody paying attention to this bug report know of a way to contact the
game developer about this?