Analysis: Make CFG::graph_iterator dereference to non-const

Since the original commit in r145858, we've had `CFG::graph_iterator`
and `CFG::const_graph_iterator`, and both have derefenced to a
`const`-ified `value_type`.  The former has an implicit conversion to
non-`const`, which is how this worked at all until r249782 started using
the dereference operator (partially reverted in r249783).

This fixes the non-const iterator to be non-const (sometimes
const-iterators are intentional, but with a separate const-ified class
(and a non-const implicit conversion leak) that's not likely to be the
case here).

llvm-svn: 249849