[LoopUnroll] Move list of params into a struct [NFCI].

Summary: Cleanup suggested in review of r358304.

Reviewers: sanjoy, efriedma

Subscribers: jlebar, zzheng, dmgreen, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60638

llvm-svn: 358723