[turbofan] Use OSR value for innermost context value.

src/compiler/linkage.h

Index: src/compiler/linkage.h
diff --git a/src/compiler/linkage.h b/src/compiler/linkage.h
index e403f63bb14ac890ded2dd295d1131538627a734..31b9faca2a9f15f145141b2160c117b6c24fb3cd 100644
--- a/src/compiler/linkage.h
+++ b/src/compiler/linkage.h
@@ -268,6 +268,9 @@ class Linkage : public ZoneObject {
   // 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_;