Add users of a MERGE_VALUE node to the worklist to process again when the node is removed.  Sorry, no test case.  Foudn it by inspection of the code

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158839 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 4b56e41..95dd2e6 100644
--- a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -1322,6 +1322,9 @@
   // Replacing results may cause a different MERGE_VALUES to suddenly
   // be CSE'd with N, and carry its uses with it. Iterate until no
   // uses remain, to ensure that the node can be safely deleted.
+  // First add the users of this node to the work list so that they
+  // can be tried again once they have new operands.
+  AddUsersToWorkList(N);
   do {
     for (unsigned i = 0, e = N->getNumOperands(); i != e; ++i)
       DAG.ReplaceAllUsesOfValueWith(SDValue(N, i), N->getOperand(i));