spirv: Make sampled images a real type

Previously, we just gave them exactly the same type as the respective
image (which already had a sampler2D or similar type).  Now they have
their own base type and a pointer to the vtn_type for the image.
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>

src/compiler/spirv/spirv_to_nir.c

@@ -511,6 +511,7 @@ vtn_type_copy(struct vtn_builder *b, struct vtn_type *src)
    case vtn_base_type_pointer:
    case vtn_base_type_image:
    case vtn_base_type_sampler:
+   case vtn_base_type_sampled_image:
       /* Nothing more to do */
       break;
 
@@ -1130,7 +1131,9 @@ vtn_handle_type(struct vtn_builder *b, SpvOp opcode,
    }
 
    case SpvOpTypeSampledImage:
-      val->type = vtn_value(b, w[2], vtn_value_type_type)->type;
+      val->type->base_type = vtn_base_type_sampled_image;
+      val->type->image = vtn_value(b, w[2], vtn_value_type_type)->type;
+      val->type->type = val->type->image->type;
       break;
 
    case SpvOpTypeSampler:

src/compiler/spirv/vtn_private.h

@@ -265,6 +265,7 @@ enum vtn_base_type {
    vtn_base_type_pointer,
    vtn_base_type_image,
    vtn_base_type_sampler,
+   vtn_base_type_sampled_image,
    vtn_base_type_function,
 };
 
@@ -347,6 +348,12 @@ struct vtn_type {
          SpvAccessQualifier access_qualifier;
       };
 
+      /* Members for sampled image types */
+      struct {
+         /* For sampled images, the image type */
+         struct vtn_type *image;
+      };
+
       /* Members for function types */
       struct {
          /* For functions, the vtn_type for each parameter */