ac: Introduce ac_build_expand()

And implement ac_bulid_expand_to_vec4() on top of it.

Fixes: 7e7ee826 ("ac: add support for 16bit buffer loads")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit 59535b05)