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.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@222978 91177308-0d34-0410-b5e6-96231b3b80d8
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.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@222979 91177308-0d34-0410-b5e6-96231b3b80d8