anv: limit compiler valid color outputs using NIR variables

This fixes a test from the vkd3d-proton test_dual_source_blending_dxbc
test which asserts in the backend with :

   brw_fs_visitor.cpp:716: void fs_visitor::emit_fb_writes(): Assertion `!prog_data->dual_src_blend || key->nr_color_regions == 1' failed.

This is because there is 2 color attachments provided by the
renderpass so we initially set nr_color_regions = 2. But once we've
parsed the shader, we can see it's only using one output (with dual
source color blending).

This change looks at the output variables to update the valid output
variables.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14417>

src/intel/vulkan/anv_pipeline.c

@@ -1190,6 +1190,24 @@ anv_pipeline_link_fs(const struct brw_compiler *compiler,
    if (deleted_output)
       nir_fixup_deref_modes(stage->nir);
 
+   /* Initially the valid outputs value is based off the renderpass color
+    * attachments (see populate_wm_prog_key()), now that we've potentially
+    * deleted variables that map to unused attachments, we need to update the
+    * valid outputs for the backend compiler based on what output variables
+    * are actually used. */
+   stage->key.wm.color_outputs_valid = 0;
+   nir_foreach_shader_out_variable_safe(var, stage->nir) {
+      if (var->data.location < FRAG_RESULT_DATA0)
+         continue;
+
+      const unsigned rt = var->data.location - FRAG_RESULT_DATA0;
+      const unsigned array_len =
+         glsl_type_is_array(var->type) ? glsl_get_length(var->type) : 1;
+      assert(rt + array_len <= MAX_RTS);
+
+      stage->key.wm.color_outputs_valid |= BITFIELD_RANGE(rt, array_len);
+   }
+
    /* We stored the number of subpass color attachments in nr_color_regions
     * when calculating the key for caching.  Now that we've computed the bind
     * map, we can reduce this to the actual max before we go into the back-end