[llvm] [MemCpyOpt] Merge memset and skip unrelated clobber in one scan (PR #90350)

[via llvm-commits]

XChy wrote:

I experiment with `std::unorderedmap` and `std::list` to speed up removing elements. But the [result](https://llvm-compile-time-tracker.com/index.php?config=Overview&stat=instructions%3Au&remote=XChy) indicates that it's not a bottleneck.

https://github.com/llvm/llvm-project/pull/90350