Shutdown/Reboot hangs Indefinitely - Ubuntu 16.04

I got exactly the same problem on Ubuntu 16.04 LTS. After installing amdgpu-pro drivers ver. 492261 system hangs at reboot. Last message printed is: Restarting System

I solved the issue by adding reboot=pci to the list of parameters passed to the kernel:

• Edit /etc/default/grub
• Add reboot=pci to the end of GRUB_CMDLINE_LINUX_DEFAULT variable
• run as root: update-grub
• reboot (system will hang, turn off/on manually)
• reboot (this time it will work)

I tried all the suggestions in the other answers and they didn't solve the issue for me.