spirv: Let spirv_to_nir() users turn sysvals into input varyings

This is an attempt at simplifying the spirv_to_nir() backend when it
comes to choosing between system values and input varyings. Let's patch
drivers to do the sysval to input varying conversion on their own so we
can get rid of the frag_coord_is_varying field in spirv_to_nir_options
and unconditionally create create sysvals for FragCoord, FrontFacing and
PointCoord inputs instead of adding new xxx_is_{sysval,varying} flags.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Suggested-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Hyunjun Ko <zzoon@igalia.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13017>

src/gallium/frontends/lavapipe/lvp_pipeline.c

@@ -529,6 +529,12 @@ lvp_shader_compile_to_ir(struct lvp_pipeline *pipeline,
 
    free(spec_entries);
 
+   const struct nir_lower_sysvals_to_varyings_options sysvals_to_varyings = {
+      .frag_coord = true,
+      .point_coord = true,
+   };
+   NIR_PASS_V(nir, nir_lower_sysvals_to_varyings, &sysvals_to_varyings);
+
    NIR_PASS_V(nir, nir_lower_variable_initializers, nir_var_function_temp);
    NIR_PASS_V(nir, nir_lower_returns);
    NIR_PASS_V(nir, nir_inline_functions);