[turbofan] Shortcut checkpoint creation in graph builder.

src/compiler/ast-graph-builder.cc

 // Copyright 2014 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/ast-graph-builder.h"
 
 #include "src/ast/scopes.h"
 #include "src/compiler.h"
 #include "src/compiler/ast-loop-assignment-analyzer.h"
 #include "src/compiler/control-builders.h"
@@ skipping 4057 matching lines @@
     DCHECK_EQ(IrOpcode::kDead,
               NodeProperties::GetFrameStateInput(node, 0)->opcode());
     bool node_has_exception = NodeProperties::IsExceptionalCall(node);
     NodeProperties::ReplaceFrameStateInput(
         node, 0,
         environment()->Checkpoint(ast_id, combine, node_has_exception));
   }
 }
 
 void AstGraphBuilder::PrepareEagerCheckpoint(BailoutId ast_id) {
+  if (environment()->GetEffectDependency()->opcode() == IrOpcode::kCheckpoint) {
+    // We skip preparing a checkpoint if there already is one the current effect
+    // dependency. This is just an optimization and not need for correctness.
+    return;
+  }
   if (ast_id != BailoutId::None()) {
     Node* node = NewNode(common()->Checkpoint());
     DCHECK_EQ(IrOpcode::kDead,
               NodeProperties::GetFrameStateInput(node, 0)->opcode());
     NodeProperties::ReplaceFrameStateInput(node, 0,
                                            environment()->Checkpoint(ast_id));
   }
 }
 
 BitVector* AstGraphBuilder::GetVariablesAssignedInLoop(
@@ skipping 299 matching lines @@
     // Phi does not exist yet, introduce one.
     value = NewPhi(inputs, value, control);
     value->ReplaceInput(inputs - 1, other);
   }
   return value;
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8