[turbofan] Handle impossible types (Type::None()) in the backend.

src/compiler/instruction-selector-impl.h

 // 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.
 
 #ifndef V8_COMPILER_INSTRUCTION_SELECTOR_IMPL_H_
 #define V8_COMPILER_INSTRUCTION_SELECTOR_IMPL_H_
 
 #include "src/compiler/instruction.h"
 #include "src/compiler/instruction-selector.h"
 #include "src/compiler/linkage.h"
@@ skipping 44 matching lines @@
   }
 
   template <typename FPRegType>
   InstructionOperand DefineAsFixed(Node* node, FPRegType reg) {
     return Define(node,
                   UnallocatedOperand(UnallocatedOperand::FIXED_FP_REGISTER,
                                      reg.code(), GetVReg(node)));
   }
 
   InstructionOperand DefineAsConstant(Node* node) {
+    return DefineAsConstant(node, ToConstant(node));
+  }
+
+  InstructionOperand DefineAsConstant(Node* node, Constant constant) {
     selector()->MarkAsDefined(node);
     int virtual_register = GetVReg(node);
-    sequence()->AddConstant(virtual_register, ToConstant(node));
+    sequence()->AddConstant(virtual_register, constant);
     return ConstantOperand(virtual_register);
   }
 
   InstructionOperand DefineAsLocation(Node* node, LinkageLocation location) {
     return Define(node, ToUnallocatedOperand(location, GetVReg(node)));
   }
 
   InstructionOperand DefineAsDualLocation(Node* node,
                                           LinkageLocation primary_location,
                                           LinkageLocation secondary_location) {
@@ skipping 336 matching lines @@
                                  // or mode_ == kFlags_set.
   BasicBlock* true_block_;       // Only valid if mode_ == kFlags_branch.
   BasicBlock* false_block_;      // Only valid if mode_ == kFlags_branch.
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_INSTRUCTION_SELECTOR_IMPL_H_