solana_rbpf::ebpf
pub const LD_DW_REG: u8 = BPF_LDX | BPF_MEM | BPF_DW; // 121u8
BPF opcode: ldxdw dst, [src + off] /// dst = (src + off) as u64.
ldxdw dst, [src + off]
dst = (src + off) as u64