Sorry I neglected to mention that I actually tried upgrading mesa earlier and it didn't resolve the problem. Neither upgrading mesa (I tried 18.3.6 and some 19.* version) nor downgrading (back to 18.3.1) would fix it, all rebuilds still resulted in artifacts. I suppose it's because the dependency on llvm 7 remained fixed during all the rebuilds.
I also just tried llvm 8.0.0 and it still had the same issue.
