[ScheduleDAGRRList] Pacify overload mismatch in std::min.

On systems where size() doesn't return unsigned long, this leads to an
overloading mismatch. Convert the constant to whatever type is used for
Q.size() on the system.

(cherry picked from commit 6c9da99)

Author: fhahn

llvm/lib/CodeGen/SelectionDAG/ScheduleDAGRRList.cpp

@@ -1841,7 +1841,8 @@ static SUnit *popFromQueueImpl(std::vector<SUnit *> &Q, SF &Picker) {
   unsigned BestIdx = 0;
   // Only compute the cost for the first 1000 items in the queue, to avoid
   // excessive compile-times for very large queues.
-  for (unsigned I = 1, E = std::min(Q.size(), 1000ul); I != E; I++)
+  for (unsigned I = 1, E = std::min(Q.size(), (decltype(Q.size()))1000); I != E;
+       I++)
     if (Picker(Q[BestIdx], Q[I]))
       BestIdx = I;
   SUnit *V = Q[BestIdx];