Remove dependence on canonical induction variable

When using the scev based code generation, we now do not rely on the presence
of a canonical induction variable any more. This commit prepares the path to
(conditionally) disable the induction variable canonicalization pass.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@177548 91177308-0d34-0410-b5e6-96231b3b80d8