aco: use v_subrev_f32 for fsub with an sgpr operand in src1
This fixes an accidentally introduced regression.
Fixes: 9be4be51 ('aco: implement 16-bit nir_op_fsub/nir_op_fadd')
This fixes an accidentally introduced regression.
Fixes: 9be4be51 ('aco: implement 16-bit nir_op_fsub/nir_op_fadd')