Remove ScheduleDAG's SUnitMap altogether. Instead, use SDNode's NodeId

Remove ScheduleDAG's SUnitMap altogether. Instead, use SDNode's NodeId
field, which is otherwise unused after instruction selection, as an index
into the SUnit array.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@52583 91177308-0d34-0410-b5e6-96231b3b80d8