A0, A1, A2, A3 opcodes aren't used

[CatsAreFluffy]

mov 0x400000, %eax is currently encoded as 8B 04 25 00 00 40 00 but can be encoded more efficiently as 67 A1 00 00 40 00. Savings are also possible for %ax and %rax using operand size prefixes, %al using A0, and stores using A2 and A3. These opcodes use a 64-bit immediate memory offset without the address-size prefix, which AFAIK no other instruction supports.