misched: Debug output fix. Use an always valid iterator.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@168767 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/MachineScheduler.cpp b/lib/CodeGen/MachineScheduler.cpp
index 8d43360..2fe72bb 100644
--- a/lib/CodeGen/MachineScheduler.cpp
+++ b/lib/CodeGen/MachineScheduler.cpp
@@ -533,7 +533,7 @@
   placeDebugValues();
 
   DEBUG({
-      unsigned BBNum = top()->getParent()->getNumber();
+      unsigned BBNum = begin()->getParent()->getNumber();
       dbgs() << "*** Final schedule for BB#" << BBNum << " ***\n";
       dumpSchedule();
       dbgs() << '\n';