Alright, so I compiled lib32-mesa-git from here
http://pkgbuild.com/~lcarlier/mesa-git/ with the two patches for radeonsi. It
doesn't crash anymore with HyperZ enabled.

I can't be 100% sure though, because Steam managed to update the game just
before I launched it. A very minor upgrade, so I assume the issue is fixed.

I guess it's time to reenable HyperZ by default, because it's the only issue I
ever had with all the games I tried so far. I will try Brütal Legend now.