[LoopInterchange] Preserve LoopInfo after interchanging.

[LoopInterchange] Preserve LoopInfo after interchanging.

LoopInterchange relies on LoopInfo being up-to-date, so we should
preserve it after interchanging. This patch updates restructureLoops to
move the BBs of the interchanged loops to the right place.

Reviewers: davide, efriedma, karthikthecool, mcrosier

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D45278

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@329264 91177308-0d34-0410-b5e6-96231b3b80d8