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.cpp

@@ -2845,7 +2845,7 @@ brw_compile_vs(const struct brw_compiler *compiler, void *log_data,
                char **error_str)
 {
    const bool is_scalar = compiler->scalar_stage[MESA_SHADER_VERTEX];
-   brw_nir_apply_sampler_key(shader, compiler, &key->tex, is_scalar);
+   brw_nir_apply_sampler_key(shader, compiler, &key->base.tex, is_scalar);
 
    const unsigned *assembly = NULL;
 
@@ -2961,7 +2961,8 @@ brw_compile_vs(const struct brw_compiler *compiler, void *log_data,
    if (is_scalar) {
       prog_data->base.dispatch_mode = DISPATCH_MODE_SIMD8;
 
-      fs_visitor v(compiler, log_data, mem_ctx, key, &prog_data->base.base,
+      fs_visitor v(compiler, log_data, mem_ctx, &key->base,
+                   &prog_data->base.base,
                    NULL, /* prog; Only used for TEXTURE_RECTANGLE on gen < 8 */
                    shader, 8, shader_time_index);
       if (!v.run_vs()) {