support GraphViz, I've been using the foo->dump() facility. This
patch is a minor rewrite to the SelectionDAG dump() stuff to make it a
little more helpful. The existing foo->dump() functionality does not
change; this patch adds foo->dumpr(). All of this is only useful when
running LLVM under a debugger.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@63736 91177308-0d34-0410-b5e6-96231b3b80d8