glsl: implement lower_packed_varyings() as a NIR pass

This is essentially the old GLSL IR packing pass rewritten as a
NIR based pass.

Doing this packing in NIR after we have preformed NIRs
optimisation passes can give us better packing results.

Acked-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15731>

src/compiler/glsl/gl_nir.h

@@ -28,7 +28,11 @@
 extern "C" {
 #endif
 
+#include <stdbool.h>
+
 struct nir_shader;
+struct gl_constants;
+struct gl_linked_shader;
 struct gl_shader_program;
 
 bool gl_nir_lower_atomics(nir_shader *shader,
@@ -44,6 +48,16 @@ bool gl_nir_lower_samplers_as_deref(nir_shader *shader,
 bool gl_nir_lower_buffers(nir_shader *shader,
                           const struct gl_shader_program *shader_program);
 
+void gl_nir_lower_packed_varyings(const struct gl_constants *consts,
+                                  struct gl_shader_program *prog,
+                                  void *mem_ctx, unsigned locations_used,
+                                  const uint8_t *components,
+                                  nir_variable_mode mode,
+                                  unsigned gs_input_vertices,
+                                  struct gl_linked_shader *linked_shader,
+                                  bool disable_varying_packing,
+                                  bool disable_xfb_packing, bool xfb_enabled);
+
 #ifdef __cplusplus
 }
 #endif

src/compiler/glsl/meson.build

@@ -86,6 +86,7 @@ files_libglsl = files(
   'gl_nir_lower_atomics.c',
   'gl_nir_lower_images.c',
   'gl_nir_lower_buffers.c',
+  'gl_nir_lower_packed_varyings.c',
   'gl_nir_lower_samplers.c',
   'gl_nir_lower_samplers_as_deref.c',
   'gl_nir_link_atomics.c',