Limited support for PHI node

[tancheng]

Currently, we can only support PHI nodes that are accompanied by CMP nodes (i.e., a problem caused by the implementation of phi node in the functional unit architecture). I might provide a new design of the PHI node in DFG in the future.
For now, it is better to manually change the following code:

  for(int x=0; x<total; ++x) {
    int i = x / maxNodeCount;
    int j = x % maxNodeCount;
    if(matrix[i][j] != 0) {
      value[index] = matrix[i][j];
      row[index] = i;
      col[index] = j;
      index++;
    }
  }

to:

  for(int x=0; x<total; ++x) {
    int i = x / maxNodeCount;
    int j = x % maxNodeCount;
    value[index] = matrix[i][j];
    row[index] = i;
    col[index] = j;
    if(matrix[i][j] != 0) {
      index++;
    }
  }

[tancheng]

Similar to the nested-loop, we now have a different way to handle ctrl flow. This problem should be fine for now.