[LoopUtils] Make duplicate method a utility. [NFCI]

Summary:
Method appendLoopsToWorklist is duplicate in LoopUnroll and in the
LoopPassManager as an internal method. Make it an utility.

Reviewers: dmgreen, chandlerc, fedor.sergeev, yamauchi

Subscribers: mehdi_amini, hiraditya, zzheng, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73569