aco: improve sub-dword emit_split_vector() with sgprs

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <mesa/mesa!4639>
24 jobs for !4639 with aco_misc_8bit_16bit_mem_ops in 8 minutes and 3 seconds (queued for 2 seconds)
merge request