spirv: Add a vtn_type field to all vtn_values

At the moment, this just lets us drop the const_type for constants and
unify things a bit.  Eventually, we will use this to store the types of
all SPIR-V SSA values.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/compiler/spirv/vtn_private.h

@@ -480,14 +480,11 @@ struct vtn_value {
    enum vtn_value_type value_type;
    const char *name;
    struct vtn_decoration *decoration;
+   struct vtn_type *type;
    union {
       void *ptr;
       char *str;
-      struct vtn_type *type;
-      struct {
-         nir_constant *constant;
-         const struct glsl_type *const_type;
-      };
+      nir_constant *constant;
       struct vtn_pointer *pointer;
       struct vtn_image_pointer *image;
       struct vtn_sampled_image *sampled_image;