[llvm] Replace compareMemory() with memcmp(); NFC (PR #126730)

[Nikita Popov via llvm-commits]

nikic wrote:

> We are not aware of any C standard library which takes advantage of this former UB, so this workaround is no longer needed.

While the standard library itself doesn't, GCC does take advantage of this, so I'm not sure this is entirely safe (yet).

https://github.com/llvm/llvm-project/pull/126730