Fix null reference creation in ScheduleDAGInstrs constructor call.

Both MachineLoopInfo and MachineDominatorTree may be null in ScheduleDAGMI
constructor call. It is undefined behavior to take references to these values.

This bug is reported by UBSan.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216118 91177308-0d34-0410-b5e6-96231b3b80d8