[IDF] Delete a redundant J-edge test

In the DJ-graph based computation of iterated dominance frontiers,
SuccNode->getIDom() == Node is one of the tests to check if (Node,Succ)
is a J-edge. If it is true, since Node is dominated by Root,

  SuccLevel = level(Node)+1 > RootLevel

which means the next test SuccLevel > RootLevel will also be true. test
the check is redundant and can be deleted as it also involves one
indirection and provides no speed-up.

llvm-svn: 355589