ac/shader: be a little smarter when scanning vertex buffers

ac/shader: be a little smarter when scanning vertex buffers

Although meta shaders don't use any vertex buffers, there is no
behaviour change but I think it's better to do this. Though,
this saves two user SGPRs for push constants inlining or
something else.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>