Speed up updateDFSNumbers with two observations:

1. domtree is a tree, not a graph.  There is no need to avoid revisiting nodes with a set.
2. the worklist can contain the child iterator pointers so we don't get N^2 rescanning of children.

This speeds up updateDFSNumbers significantly, making it basically free.  On the testcase in PR1432,
this speeds up loopsimplify by another 3x, dropping it from the 12th most expensive pass to the to
the 30th. :)  It used to be #1.

llvm-svn: 40923