Ok this solution has resolved the issue for me on a newly updated system. Note that deleting both shader caches was required, and also if the caches get rebuilt on a new generation and then I try to boot into an older previously working generation then that generation will display graphics artifacts until the caches are deleted again.
So switching between mesa compiled with llvm 6 and 7 on AMD RX 580 either backward or forward requires manually deleting the shader caches. Thanks again for the help.
