I had a somewhat similiar issue on unstable and testing - it was caused by amd-ucode package.
While my issue was fixed with a rebuild of the package - and that package should have made it to stable - you can can test if it is related by
- boot to grub - hold shift while booting
- press e to edit the highlighted entry
- add the number 3 at the end of the kernel command line
- press F10 to boot to TTY
- login as root
- remove package
pacman -R amd-ucode - run
mkinitcpio -P - run
grub-update -o /boot/grub/grub.cfg - reboot the system
If it works - 
- if it does not - it is something else