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>

src/amd/compiler/aco_validate.cpp

@@ -349,7 +349,6 @@ bool validate_ir(Program* program)
 
                check(!is_subdword || !has_const_sgpr || program->chip_class >= GFX9,
                      "Sub-dword pseudo instructions can only take constants or SGPRs on GFX9+", instr.get());
-               check(!is_subdword || !has_literal, "Sub-dword pseudo instructions cannot take literals", instr.get());
             }
 
             if (instr->opcode == aco_opcode::p_create_vector) {