src,test: fix some bugs around ivar recovery and pos splits

[rohany]

This commit fixes several bugs around the recovery of index variables
when splits/divides of the position spaces are involved. In particular,
the old code would emit position loops like

for (int jposo = A2_pos[i] / chunkSize; jposo < (A2_pos[i+1] + (chunkSize - 1)) / chunkSize; jposo++) {
  for (int jposi = 0; jposi < chunkSize; jposi++) {
    int jposA = jposo * chunkSize + jposi;
    ...
  }
}

This does not correctly iterate over the position space. A correct code is:

for (int jposo = 0; jposo < ((A2_pos[i+1] - A2_pos[i]) + (chunkSize - 1)) / chunkSize; jposo++) {
  for (int jposi = 0; jposi < chunkSize; jposi++) {
    int jposA = jposo * chunkSize + jposi + A2_pos[i];
    ...
  }
}