[polly] Fix non-determinism in polly BlockGenerators

Summary: Iterating over SeenBlocks which is a SmallPtrSet results in non-determinism in codegen

Reviewers: jdoerfert, zinob, grosser

Tags: #polly

Differential Revision: https://reviews.llvm.org/D25778

llvm-svn: 284622
GitOrigin-RevId: 5b1abfc88ec2d30801ac212b75ac1dfbcc195886