The relevant issue and bug report here (the system freezing completely or if
lucky just killing the X session, NOT games crashing) seems to be related
exclusively to AMDGPU, and not to mesa. Whereas I got the same issues over and
over after trying out several versions of mesa, switching to older versions of
the kernel "fixes" it for me (the latest version I tried out which didn't have
these issues is Kernel 4.20.13, in my case from
https://download.opensuse.org/repositories/home:/tiwai:/kernel:/4.20/standard/x86_64/)

There is also a report from another user which temporarily fixed it by forcing
the gpu to run at the maximum power setting
(https://bugzilla.opensuse.org/show_bug.cgi?id=1136293):

# echo manual > /sys/class/drm/card0/device/power_dpm_force_performance_level
# echo 7 > /sys/class/drm/card0/device/pp_dpm_sclk

and then to reset back to normal:

# echo auto > /sys/class/drm/card0/device/power_dpm_force_performance_level