aco: implement 16-bit literals

aco: implement 16-bit literals

We can copy any value into a 16-bit subregister with a 3 dword
v_pack_b32_f16 on GFX10 or a v_and_b32+v_or_b32 on GFX9.

Because the generated code can depend on the register assignment and to
improve constant propagation, Builder::copy creates a p_create_vector in
the case of sub-dword literals.

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7111>