compute dominator tree children in a deterministic order that does not depend

on the address of BasicBlock objects in memory.  This eliminates stuff like this:

 Inorder Dominator Tree:
   [1]  %entry
     [2]  %loopentry
-      [3]  %loopexit
       [3]  %no_exit
-        [4]  %endif
         [4]  %then
+        [4]  %endif
+      [3]  %loopexit
       [3]  %return

llvm-svn: 14253