[llvm] [Peephole] Check instructions from CopyMIs are still COPY (PR #69511)

[via llvm-commits]

weiguozhi wrote:

> > @mstorsjo, @mikaelholmen, is it urgent for you to fix the assertion?
> 
> Well the problem is a recent regression. I'd appreciate a quick fix, so if you plan to make larger changes taking time I would prefer a fix first. At the moment I've reverted [760e7d0](https://github.com/llvm/llvm-project/commit/760e7d00d142ba85fcf48c00e0acc14a355da7c3) downstream since we have tests that fail all the time with that commit. This works for us at the moment but living with downstream reverts always has the potential to create even more work in case something else happens upstream that depends on the reverted patch. I could of course instead apply this fix downstream, but then again, since it's a recent regression and we've identified the commit that causes it, why shouldn't it be fixed as quickly as possibly on trunk. Either by reverting or fixing.

OK, this quick fix is committed.

https://github.com/llvm/llvm-project/pull/69511