use the canonical IV only when it exists

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@177306 91177308-0d34-0410-b5e6-96231b3b80d8
ScopHelper: Remove some dead code

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@177307 91177308-0d34-0410-b5e6-96231b3b80d8
ScopHelper: clang-format

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@177309 91177308-0d34-0410-b5e6-96231b3b80d8