Cloog: Compute simple hulls during code generation

Sven suggested to use this simpler code generation strategy as the convex_hull
computation is apparently rather inefficient. We do not have a test case that
shows a difference, but, in case we find a test case where this makes a
difference, we can reconsider our decission.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@201997 91177308-0d34-0410-b5e6-96231b3b80d8