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

[Nikita Popov via llvm-commits]

nikic wrote:

Compile-time: https://llvm-compile-time-tracker.com/compare.php?from=a776740d6296520b8bde156aa3f8d9ecb32cddd9&to=d36e561ef86107c9bd0413b1d3d5fb0285ed33f7&stat=instructions:u

Notably there are regressions in stage2-O3 (but not stage1-O3), which usually indicates an optimization quality regression that affects clang itself.

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