[IslTools] dumpPw: Dump same structure pieces together.

Print same or similar structure elements together. Previously, the
value could take more importance that the space structure if visited
first in the space nest tree.

Before:
{
  Left[0] -> Right[i]: i >= 0;
  Left[1] -> AnotherRight[i];
  Left[2] -> Right[-1]
}

After:
{
  Left[0] -> Right[i]: i >= 0;
  Left[2] -> Right[-1];
  Left[1] -> AnotherRight[i]
}

llvm-svn: 322581