ScopInfo: Make getDimensionSize better reflect which dimensions carry sizes

In polly the first dimensions of an array as well as all scalars do not carry
any size information. This commit makes this explicit in the interface of
getDimensionSize. Before this commit getDimensionSize(0) returned the size of
the first dimension that carried a size. After this commit getDimensionSize(i)
will either return the size of dimension 'i' or assert in case 'i' does not
carry a size or does not exist at all.

This very same behaviour was already present in getDimensionSizePw(). This
commit also adds assertions that ensure getDimensionSizePw() is called
appropriately.

llvm-svn: 252607