[LCG] Rotate logic applied to the top of the DFSStack to instead be

[LCG] Rotate logic applied to the top of the DFSStack to instead be
applied prior to pushing a node onto the DFSStack. This is the first
step toward avoiding the stack entirely for leaf nodes. It also
simplifies things a bit and I think is pointing the way toward factoring
some more of the shared logic out of the two implementations.

It is also making it more obvious how to restructure the loops
themselves to be a bit easier to read (although no different in terms of
functionality).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207095 91177308-0d34-0410-b5e6-96231b3b80d8