MachineScheduler/ScheduleDAG: Add support to skipping a node.

The DAG mutators in the scheduler cannot really remove DAG nodes as
additional anlysis information such as ScheduleDAGToplogicalSort are
already computed at this point and rely on a fixed number of DAG nodes.

Alleviate the missing removal with a new flag: Setting the new skip
flag on a node ignores it during scheduling.

llvm-svn: 286655