OK, I just tried Mesa 18.2 from the Copr suggested by Andrew but it does not
fix the Witcher 3 for me.  Unigine Superposition seems to have been fixed by
the 4.18 kernel as I just ran it multiple times even at 4K profile and it
always finished successfully. The only thing I cannot try easily is LLVM 7
because it breaks too much dependencies on my Fedora box.