[Polly] Fully-Indexed static expansion

This commit implements the initial version of fully-indexed static
expansion.

```
 for(int i = 0; i<Ni; i++)
   for(int j = 0; j<Ni; j++)
S:     B[j] = j;
T: A[i] = B[i]
```

After the pass, we want this :
```
 for(int i = 0; i<Ni; i++)
   for(int j = 0; j<Ni; j++)
S:     B[i][j] = j;
T: A[i] = B[i][i]
```

For now we bail (fail) in the following cases:
  - Scalar access
  - Multiple writes per SAI
  - MayWrite Access
  - Expansion that leads to an access to the original array

Furthermore: We still miss checks for escaping references to the array
base pointers. A future commit will add the missing escape-checks to
stay correct in those cases. The expansion is still locked behind a
CLI-Option and should not yet be used.

Patch contributed by: Nicholas Bonfante <bonfante.nicolas@gmail.com>

Reviewers: simbuerg, Meinersbur, bollu

Reviewed By: Meinersbur

Subscribers: mgorny, llvm-commits, pollydev

Differential Revision: https://reviews.llvm.org/D34982

llvm-svn: 310304