Adjust multidim test cases to not access out-of-bound memory

We do this currently only for test cases where we have integer offsets that
clearly access array dimensions out-of-bound.

-;   for (long i = 0; i < n; i++)
-;     for (long j = 0; j < m; j++)
-;       for (long k = 0; k < o; k++)
+;   for (long i = 0; i < n - 3; i++)
+;     for (long j = 4; j < m; j++)
+;       for (long k = 0; k < o - 7; k++)
 ;         A[i+3][j-4][k+7] = 1.0;

This will be helpful if we later want to simplify the access functions under the
assumption that they do not access memory out of bounds.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@210179 91177308-0d34-0410-b5e6-96231b3b80d8