Make updateDFSNumbers API public

Make updateDFSNumbers API public

Summary:
There are a number of passes that could be sped up by using dominator tree DFS numbers to order or compare things across multiple bbs
(MemorySSA, MergedLoadStoreMotion, EarlyCSE, Sinking, GVN, NewGVN, for starters :P).

For example, GVN/CSE elimination can be done with a simple stack/etc (instead of full-on scoped hash table or repeated leader set walks)
  if the DFS pair is stored next to leaders.

The dominator tree keeps them, and the DOM tree nodes expose them as public, but you have no guarantee they are up to date (and in fact,
if you split blocks or whatever during your pass, they definitely won't be)

This means passes either have to compute their own versions[1], or make 32 queries, or ....
Rather than try to hide this, i just made the API public, and make it do nothing if the numbers are already valid.

[1] Which we want as a non-recursive walk, which is not pretty, sadly,
because it cannot use the depth first iterators since you don't get called on the way back up. So you either have to do one walk with po_iterator
and one with df_iterator, or write your own non-recursive walk that looks identical to the one in updateDFSNumbers.

Reviewers: chandlerc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8946

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234930 91177308-0d34-0410-b5e6-96231b3b80d8