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

src/compiler/instruction-selector-impl.h

Index: src/compiler/instruction-selector-impl.h
diff --git a/src/compiler/instruction-selector-impl.h b/src/compiler/instruction-selector-impl.h
index 11d66fa16b1957256f873497134659b852859dc4..83d18c3fc053af9f8f891a1a2f2da2088ee756cb 100644
--- a/src/compiler/instruction-selector-impl.h
+++ b/src/compiler/instruction-selector-impl.h
@@ -62,9 +62,13 @@ class OperandGenerator {
   }
 
   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);
   }