remove dependence on CLOOG_FOUND for PollyVectorizerChoice

When polly was configured with cmake without cloog, compilation stopped with:

../tools/polly/lib/CodeGen/BlockGenerators.cpp:662: error: 'PollyVectorizerChoice' was not declared in this scope
../tools/polly/lib/CodeGen/BlockGenerators.cpp:662: error: 'VECTORIZER_FIRST_NEED_GROUPED_UNROLL' was not declared in this scope

Merged from: https://llvm.org/svn/llvm-project/polly/trunk@168623

llvm-svn: 170736