Do allow negative offsets in the outermost array dimension

There is no needed for neither 1-dimensional nor higher dimensional arrays to
require positive offsets in the outermost array dimension.

We originally introduced this assumption with the support for delinearizing
multi-dimensional arrays.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@214665 91177308-0d34-0410-b5e6-96231b3b80d8