[NNC] API for LoopFusion

Need an API in NNC to perform loop fusion.

For example:
```
for i in 0..100:
   for j in 0..200:
     A[i,j] = sin(i*j)
for i in 0..100:
   for j in 0..199:
     B[i,j] = A[i,j] + A[i, j+1]
```
fusing the 2 outer `i` loops will result in
```
for i in 0..100:
   for j in 0..200:
     A[i,j] = sin(i*j)
   for j in 0..199:
     B[i,j] = A[i,j] + A[i, j+1]
```
