Independent blocks: SE->forget() scalars translated to arrays

This prevents SCEVs to reference values not valid any more and as a consequence
solves a bug where such values reintroduced during ast generation caused the
independent blocks pass to fail validation.

http://llvm.org/PR21204

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@222103 91177308-0d34-0410-b5e6-96231b3b80d8