In ScheduleDAGRRList::CopyAndMoveSuccessors, create the SUnit for the load

In ScheduleDAGRRList::CopyAndMoveSuccessors, create the SUnit for the load
before creating the SUnit for the operation that it was unfolded from. This
allows each SUnit to have all of its predecessor SUnits available at the time
it is created. I don't know yet if this will be absolutely required, but it
is a little tidier to do it this way.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@59083 91177308-0d34-0410-b5e6-96231b3b80d8