[isl] Simplify iterator implementation by building on top of list accessors

Summary:
With the new accessors, it's straightforward to use the templated
iterator instead of subclassing it for all the list types.

Depends on D49019

Reviewers: grosser, Meinersbur, bollu

Reviewed By: grosser

Subscribers: mehdi_amini, steven_wu, dexonsmith, pollydev, llvm-commits

Differential Revision: https://reviews.llvm.org/D49021

llvm-svn: 337702