radeonsi: if VS and TCS have the same number of threads, merge the conditonals

radeonsi: if VS and TCS have the same number of threads, merge the conditonals

Instead of:
    if (VS) {
	VS;
    }
    if (TCS) {
	TCS;
    }

Do this if the number of threads is the same in VS and TCS:
    exec = enabled_threads;
    VS;
    TCS;

Skipping declare_vb_descriptor_input_sgprs is needed to match the VS return
values.

Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7623>