i965/vec4: Handle 16-bit types at type_size_xvec4

i965/vec4: Handle 16-bit types at type_size_xvec4

These types have similar vec4 sizes as their 32-bit counterparts.

The vec4 backend doesn't support 16-bit types and probably never will,
but this method is called by the scalar backend at
fs_visitor::nir_setup_outputs(), so we still need to provide valid vec4
sizes for 16-bit types. In the future, something different should be
implemented to avoid this dependency.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>