nir/edgeflags: Add a flag to indicate the edge flag input is needed

Most modern hardware needs the edge flag added as a hidden vertex input
and needs code added to the vertex shader to copy the input to an
output.  Intel hardware is a little different.  Gfx4 and Gfx5 hardware
works in the previously described mannter.  Gfx6+ hardware needs the
edge flag as a specific vertex shader input, and that input is magically
processed by fixed-function hardware without need for extra shader code.

This flag signals only that the vertex shader input is needed.  It would
be nice if we could decouple adding the vertex shader input from
generating the copy-to-output code, but that has proven to be
challenging.  Not having that code causes other passes to want to
eliminate that shader input.

v2: Convert conditional to assertion.  This pass is only called for
vertex shaders.  Suggested by Ken.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12858>

src/compiler/nir/nir_lower_passthrough_edgeflags.c

@@ -96,5 +96,9 @@ lower_impl(nir_function_impl *impl)
 
 void nir_lower_passthrough_edgeflags(nir_shader *shader)
 {
+   assert(shader->info.stage == MESA_SHADER_VERTEX);
+
+   shader->info.vs.needs_edge_flag = true;
+
    lower_impl(nir_shader_get_entrypoint(shader));
 }

src/compiler/shader_info.h

@@ -297,6 +297,9 @@ typedef struct shader_info {
 
          /* True if the shader writes position in window space coordinates pre-transform */
          bool window_space_position:1;
+
+         /** Is an edge flag input needed? */
+         bool needs_edge_flag:1;
       } vs;
 
       struct {

src/gallium/drivers/iris/iris_program.c

@@ -2441,6 +2441,7 @@ iris_create_uncompiled_shader(struct iris_screen *screen,
    simple_mtx_init(&ish->lock, mtx_plain);
 
    NIR_PASS(ish->needs_edge_flag, nir, iris_fix_edge_flags);
+   assert(ish->needs_edge_flag == nir->info.vs.needs_edge_flag);
 
    brw_preprocess_nir(screen->compiler, nir, NULL);