aco: allow GFX9 partial writes with instructions which use opsel

Some instructions such as v_mad_f16 can do partial writes on GFX9.

No fossil-db changes.

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/5245>

src/amd/compiler/aco_register_allocation.cpp

@@ -429,7 +429,7 @@ std::pair<unsigned, unsigned> get_subdword_definition_info(Program *program, con
    if (can_use_SDWA(chip, instr)) {
       return std::make_pair(rc.bytes(), rc.bytes());
    } else if (rc.bytes() == 2 && can_use_opsel(chip, instr->opcode, -1, 1)) {
-      return std::make_pair(2u, chip >= GFX10 ? 2u : 4u);
+      return std::make_pair(2u, bytes_written);
    }
 
    switch (instr->opcode) {