ac/nir: only use the new raw/struct image atomic intrinsics with LLVM 9+

They are buggy with LLVM 8 because they weren't marked as source
of divergence, see r358579.

Fixes: dd0172e8 ("radv: Use structured intrinsics instead of indexing workaround for GFX9.")"
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
(cherry picked from commit 31164cf5)