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

[via llvm-commits]

https://github.com/dianqk commented:

Possible miscompile: https://llvm-compile-time-tracker.com/compare.php?from=2f66e5fcbad211e0d71b1a533071d31b36f088ec&to=0d828e6dcb86d886f1e5ee57ec7e71b945bfd36c&stat=instructions:u

Handle the there cases of `Offset` is difficult to read. How about splitting into two parts (with two PRs)?

- If it's an undefined value, trim the start and end
- Handle the case where Offset >= 0 here

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