SchedDFS: Constify interface.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173398 91177308-0d34-0410-b5e6-96231b3b80d8
SchedDFS: Refactor and tweak the subtree selection criteria.

For sanity, create a root when NumDataSuccs >= 4. Splitting large
subtrees will no longer be detrimental after my next checkin to handle
nested tree. A magic number of 4 is fine because single subtrees
seldom rejoin more than this. It makes subtrees easier to visualize
and heuristics more sane.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173399 91177308-0d34-0410-b5e6-96231b3b80d8