SchedDFS: Complete support for nested subtrees.

Maintain separate per-node and per-tree book-keeping.
Track all instructions above a DAG node including nested subtrees.
Seperately track instructions within a subtree.
Record subtree parents.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173426 91177308-0d34-0410-b5e6-96231b3b80d8
ScheduleDAG: Added isBoundaryNode to conveniently detect a common corner case.

This fixes DAG subtree analysis at the boundary.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173427 91177308-0d34-0410-b5e6-96231b3b80d8