st/glsl_to_nir: gather next_stage in shader_info

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/compiler/shader_info.h

@@ -64,6 +64,11 @@ typedef struct shader_info {
    /** The shader stage, such as MESA_SHADER_VERTEX. */
    gl_shader_stage stage;
 
+   /** The shader stage in a non SSO linked program that follows this stage,
+     * such as MESA_SHADER_FRAGMENT.
+     */
+   gl_shader_stage next_stage;
+
    /* Number of textures used by this shader */
    unsigned num_textures;
    /* Number of uniform buffers used by this shader */