misched: Infrastructure for weak DAG edges.

This adds support for weak DAG edges to the general scheduling
infrastructure in preparation for MachineScheduler support for
heuristics based on weak edges.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@167738 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SelectionDAG/ScheduleDAGVLIW.cpp

@@ -123,6 +123,8 @@ void ScheduleDAGVLIW::releaseSucc(SUnit *SU, const SDep &D) {
     llvm_unreachable(0);
   }
 #endif
+  assert(!D.isWeak() && "unexpected artificial DAG edge");
+
   --SuccSU->NumPredsLeft;
 
   SuccSU->setDepthToAtLeast(SU->getDepth() + D.getLatency());