[llvm] [NFC] Extract LoopConstrainer from IRCE to reuse it outside the pass (PR #70508)

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

fyi, there's no need to touch gn files if you're not using them (this patch actually broke the gn build and the gn file list auto-updater due to a missing comma)

https://github.com/llvm/llvm-project/pull/70508