[polly] Fix non-deterministic output due to iteration of unordered ScopArrayInfo

Summary:
This fixes the following failures in the reverse iteration builder:
http://lab.llvm.org:8011/builders/reverse-iteration/builds/25

    Polly :: MaximalStaticExpansion/working_deps_between_inners.ll
    Polly :: MaximalStaticExpansion/working_expansion_multiple_dependences_per_statement.ll
    Polly :: MaximalStaticExpansion/working_expansion_multiple_instruction_per_statement.ll
    Polly :: MaximalStaticExpansion/working_phi_expansion.ll

Reviewers: simbuerg, Eugene.Zelenko, grosser, zinob, bollu

Reviewed By: grosser

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37349

llvm-svn: 312273