[llvm] [M68k] Fix encoding of CAS instructions (PR #154481)

[via llvm-commits]

knickish wrote:

this one is confusing, as the CAS section of the reference doesn't talk about extension words at all, but the addressing modes it supports clearly require them.

<img width="790" height="282" alt="image" src="https://github.com/user-attachments/assets/4f9b6648-8d7d-4409-ad1b-11938b057edf" />


> 2.2.17 Absolute Long Addressing Mode
> In this addressing mode, the operand is in memory, and the operand’s address occupies the two extension words following the instruction word in memory. The first extension word contains the high-order part of the address; the second contains the low-order part of the address.


I think this must be the correct way to encode them



https://github.com/llvm/llvm-project/pull/154481