compiler: tidy up double_inputs_read uses

First we move double_inputs_read into a vs struct in the union,
double_inputs_read is only used for vs inputs so this will
save space and also allows us to add a new double_inputs field.

We add the new field because c2acf97fcc changed the behaviour
of double_inputs_read, and while it's no longer used to track
actual reads in i965 we do still want to track this for gallium
drivers.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/intel/compiler/brw_vec4.cpp

@@ -2769,7 +2769,7 @@ brw_compile_vs(const struct brw_compiler *compiler, void *log_data,
    }
 
    prog_data->inputs_read = shader->info.inputs_read;
-   prog_data->double_inputs_read = shader->info.double_inputs_read;
+   prog_data->double_inputs_read = shader->info.vs.double_inputs;
 
    brw_nir_lower_vs_inputs(shader, key->gl_attrib_wa_flags);
    brw_nir_lower_vue_outputs(shader, is_scalar);