[llvm] [M68k] Prevent COPY instruction from killing live condition flags (PR #168485)

[Peter Mackay via llvm-commits]

petmac wrote:

If possible I’d still like to explore alternative options to fix the clobbering.

At the moment it does:
- Back up CCR (in a very CPU model-specific way)
- Do the clobbering MOVE
- Restore the CCR

What I would maybe prefer if it’s possible:
- Don’t back up the CCR
- Do an equivalent MOVE in a way that doesn’t clobber the CCR (MOVEMs to/from stack or maybe even something better if it exists)

https://github.com/llvm/llvm-project/pull/168485