ASM_FMT_B4
op = ASM_OP_BR_COND, fmt = ASM_FMT_B4;
op = ASM_OP_BR_IA, fmt = ASM_FMT_B4;
op = ASM_OP_BR_RET, fmt = ASM_FMT_B4;
case ASM_FMT_B4: /* 0 dst */