[llvm] [clang-tools-extra] [clang] Move ExpandMemCmp and MergeIcmp to the middle end (PR #77370)

[Nikita Popov via cfe-commits]

nikic wrote:

@gbaraldi New compile-time numbers: https://llvm-compile-time-tracker.com/compare.php?from=ecd47811b755d13357085bcd7519a66d6c4d8e5c&to=f582b874c35e2b90046da8e8c30a7fa30ba08167&stat=instructions:u (They are mildly positive now, and there are some text size reductions.)

https://github.com/llvm/llvm-project/pull/77370