Previously GraphWriter assumed operator*() for node_iterator always

Previously GraphWriter assumed operator*() for node_iterator always
returned a reference type.  This patch allows operator*() to return a
non-reference type while still maintaining the old behavior when it
does return a reference type.

This patch was motivated when I tried to use "df_iterator" (see
llvm/ADT/DepthFirstIterator.h) as a "node_iterator", as df_iterator
does not return a reference type and thus we would get a compilation
error when trying to take the address of a temporary.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@42151 91177308-0d34-0410-b5e6-96231b3b80d8