MIsched: Added biasCriticalPath.

Allow schedulers to order DAG edges by critical path. This makes
DFS-based heuristics more stable and effective.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173317 91177308-0d34-0410-b5e6-96231b3b80d8
MachineScheduler: enable biasCriticalPath for all DAGs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173318 91177308-0d34-0410-b5e6-96231b3b80d8