solana_rbpf::ebpf
pub const LD_B_REG: u8 = BPF_LDX | BPF_MEM | BPF_B; // 113u8
BPF opcode: ldxb dst, [src + off] /// dst = (src + off) as u8.
ldxb dst, [src + off]
dst = (src + off) as u8