[llvm] [NFC][M68k] Refactor away MOV8dc and MOV8cd (PR #169426)

[Daniel Thornburgh via llvm-commits]

mysterymath wrote:

> You mentioned in the linked issue that you encountered a bug related to this while working on llvm-libc for m68k, can you add a test that includes a minimal repro of whatever failure you hit?

This is truly a NFC to make it easier to make a follow up change that fixes that issue; that includes a test. I didn't think to do the whole stacked PR thing this time. I can add the test if necessary disabled by default (as it will presently fail), but it seems cleaner to introduce it alongside the fix.

https://github.com/llvm/llvm-project/pull/169426