[turbofan] Use OSR value for innermost context value.

src/compiler/linkage.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_LINKAGE_H_
 #define V8_COMPILER_LINKAGE_H_
 
 #include "src/base/flags.h"
 #include "src/compiler/frame.h"
 #include "src/compiler/machine-type.h"
@@ skipping 250 matching lines @@
   FrameOffset GetFrameOffset(int spill_slot, Frame* frame, int extra = 0) const;
 
   static int FrameStateInputCount(Runtime::FunctionId function);
 
   // Get the location where an incoming OSR value is stored.
   LinkageLocation GetOsrValueLocation(int index) const;
 
   // A special parameter index for JSCalls that represents the closure.
   static const int kJSFunctionCallClosureParamIndex = -1;
 
+  // A special {OsrValue} index to indicate the context spill slot.
+  static const int kOsrContextSpillSlotIndex = -1;
+
  private:
   CallDescriptor* const incoming_;
 
   DISALLOW_COPY_AND_ASSIGN(Linkage);
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_LINKAGE_H_