Replace custom written DFS walk with depth first iterator

Summary:
GenericDomTreeConstruction had its own written DFS walk.
It is basically identical to the DFS walk df_* is doing in the iterators.
the one difference is that df_iterator uses an internal visited set.
The GenericDomTreeConstruction one reused a field in an existing densemap lookup.

Time-wise, this way is actually more cache-friendly (the previous way has a random store
into a successor's info, the new way does that store at the same time and in the same place
as other stores to the same info)

It costs some very small amount of memory to do this, and one we pay in some other part of
dom tree construction *anyway*, so we aren't really increasing dom tree constructions's
peak memory usage.

It could still be changed to use the old field with a little work on df_ext_* if we care
(and if someone find performance regressions)

Reviewers: chandlerc

Reviewed By: chandlerc

Subscribers: Eugene.Zelenko, llvm-commits

Differential Revision: https://reviews.llvm.org/D8932

llvm-svn: 294339