spirv: Sort out the mess that is sampled image

This MR 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.