broadcom/compiler: track if the fragment shader forces per-sample MSAA

For example, regarding gl_SampleID, the GLSL spec states:

   "Any static use of this variable in a fragment shader causes the
    entire shader to be evaluated per-sample."

So we need to track if the fragment shader does anything that implicitly
enables per-sample shading in the compiler for the driver to
auto-enable sample rate shading if needed.

v2:
 - Instead of tracking reads of gl_SampleID, check SYSTEM_BIT_SAMPLE_ID
   and SYSTEM_BIT_SAMPLE_POS as well as the sample layout qualifier like
   other drivers are doing to activate this behavior (Eric).

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com> (v1)
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6766>

src/broadcom/compiler/nir_to_vir.c

@@ -2827,6 +2827,11 @@ nir_to_vir(struct v3d_compile *c)
                         c->line_x = emit_fragment_varying(c, NULL, 0, 0);
                         c->uses_implicit_point_line_varyings = true;
                 }
+
+                c->force_per_sample_msaa =
+                   c->s->info.fs.uses_sample_qualifier ||
+                   (c->s->info.system_values_read & (SYSTEM_BIT_SAMPLE_ID |
+                                                     SYSTEM_BIT_SAMPLE_POS));
                 break;
         case MESA_SHADER_COMPUTE:
                 /* Set up the TSO for barriers, assuming we do some. */

src/broadcom/compiler/v3d_compiler.h

@@ -571,6 +571,11 @@ struct v3d_compile {
         bool writes_z;
         bool uses_implicit_point_line_varyings;
 
+        /* If the fragment shader does anything that requires to force
+         * per-sample MSAA, such as reading gl_SampleID.
+         */
+        bool force_per_sample_msaa;
+
         /* Whether we are using the fallback scheduler. This will be set after
          * register allocation has failed once.
          */
@@ -798,6 +803,7 @@ struct v3d_fs_prog_data {
         bool uses_center_w;
         bool uses_implicit_point_line_varyings;
         bool lock_scoreboard_on_first_thrsw;
+        bool force_per_sample_msaa;
 };
 
 struct v3d_compute_prog_data {

src/broadcom/compiler/vir.c

@@ -760,6 +760,7 @@ v3d_fs_set_prog_data(struct v3d_compile *c,
                 c->uses_implicit_point_line_varyings;
         prog_data->lock_scoreboard_on_first_thrsw =
                 c->lock_scoreboard_on_first_thrsw;
+        prog_data->force_per_sample_msaa = c->force_per_sample_msaa;
 }
 
 static void