[turbofan] Unify referencing of stack slots

src/frames.h

Index: src/frames.h
diff --git a/src/frames.h b/src/frames.h
index 9eee5a136e9b8d35cfa61215b7a06afb76e6e092..1d64c747103536840381b3018ab0085fac100b29 100644
--- a/src/frames.h
+++ b/src/frames.h
@@ -120,8 +120,9 @@ class StandardFrameConstants : public AllStatic {
   static const int kCPSlotSize =
       FLAG_enable_embedded_constant_pool ? kPointerSize : 0;
   static const int kFixedFrameSizeFromFp =  2 * kPointerSize + kCPSlotSize;
+  static const int kFixedFrameSizeAboveFp = kPCOnStackSize + kFPOnStackSize;
   static const int kFixedFrameSize =
-      kPCOnStackSize + kFPOnStackSize + kFixedFrameSizeFromFp;
+      kFixedFrameSizeAboveFp + kFixedFrameSizeFromFp;
   static const int kExpressionsOffset = -3 * kPointerSize - kCPSlotSize;
   static const int kMarkerOffset = -2 * kPointerSize - kCPSlotSize;
   static const int kContextOffset = -1 * kPointerSize - kCPSlotSize;
@@ -678,6 +679,8 @@ class OptimizedFrame : public JavaScriptFrame {
 
   DeoptimizationInputData* GetDeoptimizationData(int* deopt_index);
 
+  static int StackSlotOffsetRelativeToFp(int slot_index);
+
  protected:
   inline explicit OptimizedFrame(StackFrameIteratorBase* iterator);