i965/nir: add a helper to lower gl_PatchVerticesIn to a uniform

v2: do not try to handle it as a system value directly for the SPIR-V
    path. In GL we rather handle it as a uniform like we do for the
    GLSL path (Jason).

v3:
  - Remove the uniform variable, it is alwats -1 now (Jason)
  - Also do the lowering for the TessEval stage (Jason)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/intel/compiler/brw_nir.h

@@ -145,6 +145,8 @@ void brw_nir_setup_arb_uniforms(void *mem_ctx, nir_shader *shader,
                                 struct gl_program *prog,
                                 struct brw_stage_prog_data *stage_prog_data);
 
+void brw_nir_lower_patch_vertices_in_to_uniform(nir_shader *nir);
+
 void brw_nir_analyze_ubo_ranges(const struct brw_compiler *compiler,
                                 nir_shader *nir,
                                 struct brw_ubo_range out_ranges[4]);