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

src/compiler/scheduler.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/scheduler.h"
 
 #include <iomanip>
 
 #include "src/base/adapters.h"
 #include "src/bit-vector.h"
@@ skipping 1520 matching lines @@
                         : BasicBlock::GetCommonDominator(block, use_block);
     }
     return block;
   }
 
   BasicBlock* FindPredecessorBlock(Node* node) {
     return scheduler_->control_flow_builder_->FindPredecessorBlock(node);
   }
 
   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
       // dominator of its uses. This will not recurse more than one level.
       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
       // predecessor block of the corresponding control input to the merge.
       if (scheduler_->GetPlacement(use) == Scheduler::kFixed) {
         TRACE("  input@%d into a fixed phi #%d:%s\n", edge.index(), use->id(),
               use->op()->mnemonic());
         Node* merge = NodeProperties::GetControlInput(use, 0);
         DCHECK(IrOpcode::IsMergeOpcode(merge->opcode()));
         Node* input = NodeProperties::GetControlInput(merge, edge.index());
@@ skipping 175 matching lines @@
   for (Node* const node : *nodes) {
     schedule_->SetBlockForNode(to, node);
     scheduled_nodes_[to->id().ToSize()].push_back(node);
   }
   nodes->clear();
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8