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

[via llvm-commits]

paperchalice wrote:

BTW, the implementation I envisioned in https://discourse.llvm.org/t/rfc-codegen-new-pass-manager-pipeline-construction-design/84659/5 also need this.

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