Allow overflow of indices with constant dim-sizes.

Allow overflow of indices into the next higher dimension if it has
constant size. E.g.

    float A[32][2];
    ((float*)A)[5];

is effectively the same as

    A[2][1];

This can happen since r265379 as a side effect if ScopDetection
recognizes an access as affine, but ScopInfo rejects the GetElementPtr.

Differential Revision: http://reviews.llvm.org/D18878

llvm-svn: 265942