[ScopInfo] Move foldSizeConstantsToRight() to isl++

Summary: This patch updates the isl interface used in `foldSizeConstantsToRight()` to the new C++ interface.

Reviewers: chelini, grosser, philip.pfaffe, Meinersbur

Reviewed By: grosser

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D48965

llvm-svn: 336362