Use "const auto &" in range based for loops

This previous code added in r216842 most likely created unnecessary copies.

Reported-by: Duncan P. N. Exon Smith  <dexonsmith@apple.com>

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@217507 91177308-0d34-0410-b5e6-96231b3b80d8