Some applications (e.g. clinfo) requires user to be participant of 'video' group. Instruction for 17.40 contains adding user to this group as a step, 17.50 one (How-To Install/Uninstall AMD Radeon™ Software AMDGPU-PRO Driver for Linux® on an Ubuntu System ) doesn't.
How did you install 17.50 driver? I've tried that instructions, but there is no '/opt/amdgpu-pro/bin' folder after installation, where the variable 'LLVM_BIN' points to. So I don't know how to run 'clinfo' command. I also tried to install 'opencl-amdgpu-pro' package, but it contains only changelog and copyright files.