intel/compiler: Add a "base class" for program keys

Right now, all keys have two things in common: a program string ID and a
sampler_prog_key_data.  I'd like to add another thing or two and need a
place to put it.  This commit adds a new brw_base_prog_key struct which
contains those two common bits.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/intel/compiler/brw_vec4_tes.cpp

@@ -40,7 +40,7 @@ vec4_tes_visitor::vec4_tes_visitor(const struct brw_compiler *compiler,
                                   const nir_shader *shader,
                                   void *mem_ctx,
                                   int shader_time_index)
-   : vec4_visitor(compiler, log_data, &key->tex, &prog_data->base,
+   : vec4_visitor(compiler, log_data, &key->base.tex, &prog_data->base,
                   shader, mem_ctx, false, shader_time_index)
 {
 }