[llvm] [PassManager][Loop] Merge loop pass manager when add it (PR #135150)

[Arthur Eubanks via llvm-commits]

https://github.com/aeubanks commented:

makes sense, although I think this is deserving of a test, perhaps a unittest where we do `addPass()` on another LPM and then dump the pipeline as a string to make sure we don't see something like `loop(loop(...))`

https://github.com/llvm/llvm-project/pull/135150