[polly] Change SmallPtrSet which is being iterated to SmallSetVector in ScopInfo.h

Summary: This will avoid non-deterministic iteration order.

Reviewers: grosser, jdoerfert, zinob, mgrang

Subscribers: #polly

Tags: #polly

Differential Revision: https://reviews.llvm.org/D25880

llvm-svn: 284883