[Polly][isl] Add neutrally-named accessors to isl list elements and sizes

Summary: This could simplify the isl iterator implementation a lot.

Reviewers: grosser, Meinersbur, bollu

Reviewed By: grosser

Subscribers: pollydev, llvm-commits

Differential Revision: https://reviews.llvm.org/D49019

llvm-svn: 337054