spirv: Sort out the mess that is sampled image

spirv: Sort out the mess that is sampled image

This commit makes two major changes.  First, we add a second case to
OpLoad for sampled images which constructs a vtn_sampled_image and
stashes that rather than stashing a pointer to the combined image
sampler like we do for bare samplers and images.  This should be more in
line with how SPIR-V is intended to work and hopefully doesn't cause any
weird problems.  The second is a rework of vtn_handle_texture to assume
that everything has an image but not everything has a sampler.  We also
add a vtn_fail_if for the case where a texture instructions require a
sampler but none is provided.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>