spirv: Rework our handling of images and samplers

spirv: Rework our handling of images and samplers

Previously, objects of type OpTypeImage or OpTypeSampler were treated as
vtn_pointers and objects of type OpTypeSampledImage were a special-use
vtn_sampled_image struct.  This commit changes that so that all of those
objects are stored in vtn_ssa_values.  Each of images, samplers, and
sampled images, are stored as a scalar or vector nir_ssa_def whose
components are NIR deref values.  We now use vtn_type_get_nir_type to
re-resolve those as-needed into GLSL sampler types for NIR.

This simplification has a number of benefits:

 1. We can git rid of the rest of our special-cases for handling images
    and samplers in function arguments.  Now that they're treated as
    structs at the glsl_type level, the generic paths can handle images
    and samplers.

 2. We can now construct composite values containing images and samplers
    internally.  It's unclear from the SPIR-V spec whether or not this
    is allowed and it's not a pattern that GLSLang currently generates
    thanks to GLSL rules.  However, if we do start seeing SPIR-V that
    contains such composites, we should now be able to handle it.

 3. SPIR-V OpNull and OpUndef instructions can now create samplers,
    images, and sampled images.  The NIR generated won't likely be fully
    valid but, given a NIR pass to do something sensible, it should be a
    thing we can compile.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5278>