[Polly] Fix for the JSON Exporter

Summary:
Small patch to fix the JSON exporter.

Currently, using "opt -polly-export-jscop" does not generate jscop files, but gives an error:
*** Error in `opt': corrupted double-linked list: 0x0000000000bc4bb0 ***

Updated the function getAccessRelationStr() to work with the current version of getAccessRelation(), fixing the JSON exporter

Reviewers: bollu, grosser

Reviewed By: grosser

Subscribers: grosser, llvm-commits, pollydev

Tags: #polly

Differential Revision: https://reviews.llvm.org/D36370

llvm-svn: 310199