[llvm] [memcpyopt] handle memcpy from memset in more cases (PR #140954)

[via llvm-commits]

https://github.com/dianqk commented:

> I don't have merge rights anymore (I wasn't active enough when they were reviewing the list of people with access, though I can ask for it to be reinstated if you'd prefer that I merge)

Feel free to request :)
And I can merge it after your response.

https://github.com/llvm/llvm-project/pull/140954