intel/compiler: fix generation of vec8/vec16 alu instruction

intel/compiler: fix generation of vec8/vec16 alu instruction

I stumbled on this when I inserted some suboptimal lowering code after all
optimizations. Adding certain subset of optimizations after my lowering code
actually avoided this bug, so I think it's not possible to hit this on upstream.

Let's fix this for the next person generating suboptimal code...

Reviewed-by: Sagar Ghuge <sagar.ghuge@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20857>