[llvm] [M68k] Finish implementation of `MOVX` (move and extend) pseudo-instructions and fix related errors (PR #218938)

[via llvm-commits]

https://github.com/knickish approved this pull request.

This looks good to me in general, although I'm concerned that I don't know this well enough to give good feedback (@mshockwave would you be able to take a look here?)

Also, does this let us turn on machine verifier for any more tests now? You talked a bit about it in the pr text, but don't see it turned on in any additional tests

https://github.com/llvm/llvm-project/pull/218938