[turbofan] Unify referencing of stack slots

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
diff --git a/src/x64/lithium-codegen-x64.cc b/src/x64/lithium-codegen-x64.cc
index 2b5def39c1050fe1d8f4d443a8e2f470e232fc6f..0398d7f5d1f1bcb5444ff38546ea322b2cf29524 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -592,15 +592,23 @@ void LCodeGen::AddToTranslation(LEnvironment* environment,
   }
 
   if (op->IsStackSlot()) {
+    int index = op->index();
+    if (index >= 0) {
+      index += StandardFrameConstants::kFixedFrameSize / kPointerSize;
+    }
     if (is_tagged) {
-      translation->StoreStackSlot(op->index());
+      translation->StoreStackSlot(index);
     } else if (is_uint32) {
-      translation->StoreUint32StackSlot(op->index());
+      translation->StoreUint32StackSlot(index);
     } else {
-      translation->StoreInt32StackSlot(op->index());
+      translation->StoreInt32StackSlot(index);
     }
   } else if (op->IsDoubleStackSlot()) {
-    translation->StoreDoubleStackSlot(op->index());
+    int index = op->index();
+    if (index >= 0) {
+      index += StandardFrameConstants::kFixedFrameSize / kPointerSize;
+    }
+    translation->StoreDoubleStackSlot(index);
   } else if (op->IsRegister()) {
     Register reg = ToRegister(op);
     if (is_tagged) {