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

[Jameson Nash via llvm-commits]

vtjnash wrote:

Review request bump? I have some other improvements I'd like to propose (https://github.com/llvm/llvm-project/compare/main...vtjnash:llvm-project:jn/memcpyopt-destonly), so this PR was a bit of a practice exercise for me to learn the optimization pass

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