spirv: Add an option for making FragCoord a sysval

On AMD, FragCoord should be a sysval because it is handled separately
from all the other inputs. We were already doing this in radeonsi, but
we weren't doing it with radv. It'll be much more annoying to handle
VARYING_SLOT_POS in fragment shaders when we let NIR lower FS inputs for
us, so here we add an option so that radv can get it as a system value.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>

src/compiler/spirv/nir_spirv.h

@@ -70,6 +70,11 @@ struct spirv_to_nir_options {
    /* Whether or not to lower all UBO/SSBO access to offsets up-front. */
    bool lower_ubo_ssbo_access_to_offsets;
 
+   /* Whether to make FragCoord to a system value, the same as
+    * GLSLFragCoordIsSysVal in GLSL.
+    */
+   bool frag_coord_is_sysval;
+
    struct spirv_supported_capabilities caps;
 
    /* Address format for various kinds of pointers. */

src/compiler/spirv/vtn_variables.c

@@ -1295,8 +1295,13 @@ vtn_get_builtin_location(struct vtn_builder *b,
       set_mode_system_value(b, mode);
       break;
    case SpvBuiltInFragCoord:
-      *location = VARYING_SLOT_POS;
       vtn_assert(*mode == nir_var_shader_in);
+      if (b->options && b->options->frag_coord_is_sysval) {
+         *mode = nir_var_system_value;
+         *location = SYSTEM_VALUE_FRAG_COORD;
+      } else {
+         *location = VARYING_SLOT_POS;
+      }
       break;
    case SpvBuiltInPointCoord:
       *location = VARYING_SLOT_PNTC;