Enable postordering when computing the sparse factorization.

Previously when using a natural ordering, we had postordering
turned off. This is not a good idea. Enabling postordering will
also has the possibility of improving the size of the supernodes.

Change-Id: I8c270e54751b8bed53b38a0b461f647f5c8f5640

internal/ceres/suitesparse.cc

diff --git a/internal/ceres/suitesparse.cc b/internal/ceres/suitesparse.cc
index 8f7a4c7..88ef935 100644
--- a/internal/ceres/suitesparse.cc
+++ b/internal/ceres/suitesparse.cc
@@ -166,18 +166,8 @@
                                              string* message) {
   cc_.nmethods = 1;
   cc_.method[0].ordering = OrderingTypeToCHOLMODEnum(ordering_type);
-  cc_.supernodal = CHOLMOD_AUTO;
-
-  // TODO(sameeragarwal): The following modification exists entirely
-  // to preserve existing behaviour. It is not clear if this is
-  // actually the optimal thing to do. Once I understand the use of
-  // postordering better, I will come back and document this/remove it
-  // as the case maybe.
-  if (ordering_type == OrderingType::NATURAL) {
-    cc_.postorder = 0;
-  }
-
   cholmod_factor* factor = cholmod_analyze(A, &cc_);
+
   if (cc_.status != CHOLMOD_OK) {
     *message =
         StringPrintf("cholmod_analyze failed. error code: %d", cc_.status);
@@ -198,9 +188,9 @@
 
   cc_.nmethods = 1;
   cc_.method[0].ordering = CHOLMOD_GIVEN;
-
   cholmod_factor* factor =
       cholmod_analyze_p(A, const_cast<int*>(&ordering[0]), nullptr, 0, &cc_);
+
   if (cc_.status != CHOLMOD_OK) {
     *message =
         StringPrintf("cholmod_analyze failed. error code: %d", cc_.status);