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