Remove -polly-codegen-scev option and related code

SCEV based code generation has been the default for two weeks after having
been tested for a long time. We now drop the support the non-scev-based code
generation.
Remove Polly's IndVarSimplify pass

Polly had a copy of this pass to create the canonical induction variables
necessary for the non-scev-based code generation. As we now always use SCEV
based code generation, canonical induction variables are not needed any more.