intel/vec4: Rework texture handling to not use `ir_texture_opcode`
When we built the vec4 NIR back-end, we made the questionable decision to share texture handling with the GLSL IR back-end we had at the time. That meant translating NIR texure ops to look a bit more like GLSL IR ones and calling the same helper function from both. The GLSL IR back-end died several years ago and won't be coming back. We should roll it all into brw_vec4_nir.cpp and drop the translation.