[ADT] Actually mutate the iterator VisitStack.back().second, not its copy.

Summary: Before the change, *Opt never actually gets updated by the end
of toNext(), so for every next time the loop has to start over from
child_begin(). This bug doesn't affect the correctness, since Visited prevents
it from re-entering the same node again; but it's slow.

Reviewers: dberris, dblaikie, dannyb

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D23649

llvm-svn: 279482