[turbofan] Handle dead diamonds in scheduling and add a test.

src/compiler/scheduler.cc

Index: src/compiler/scheduler.cc
diff --git a/src/compiler/scheduler.cc b/src/compiler/scheduler.cc
index 80ce8b17112da8404f14fd5ed5db0a2c3f49eacf..b04ba6f92636feb93a3e4e629d4166f92fb64de9 100644
--- a/src/compiler/scheduler.cc
+++ b/src/compiler/scheduler.cc
@@ -1538,6 +1538,8 @@ class ScheduleLateNodeVisitor {
   }
 
   BasicBlock* GetBlockForUse(Edge edge) {
+    // TODO(titzer): ignore uses from dead nodes (not visited in PrepareUses()).
+    // Dead uses only occur if the graph is not trimmed before scheduling.
     Node* use = edge.from();
     if (IrOpcode::IsPhiOpcode(use->opcode())) {
       // If the use is from a coupled (i.e. floating) phi, compute the common
@@ -1545,7 +1547,8 @@ class ScheduleLateNodeVisitor {
       if (scheduler_->GetPlacement(use) == Scheduler::kCoupled) {
         TRACE("  inspecting uses of coupled #%d:%s\n", use->id(),
               use->op()->mnemonic());
-        DCHECK_EQ(edge.to(), NodeProperties::GetControlInput(use));
+        // TODO(titzer): reenable once above TODO is addressed.
+        //        DCHECK_EQ(edge.to(), NodeProperties::GetControlInput(use));
         return GetCommonDominatorOfUses(use);
       }
       // If the use is from a fixed (i.e. non-floating) phi, we use the