Fix DoubleStackSlot-to-DoubleStackSlot moves on ia32. Unify platform-independent code.

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 f71bf38e42a0ecae4453f12dd29b9a018c5c550d..b3c4bc827d8f98ca76ea6d0427167a9137271cfd 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -464,15 +464,7 @@ Operand LCodeGen::ToOperand(LOperand* op) const {
   // Does not handle registers. In X64 assembler, plain registers are not
   // representable as an Operand.
   ASSERT(op->IsStackSlot() || op->IsDoubleStackSlot());
-  int index = op->index();
-  if (index >= 0) {
-    // Local or spill slot. Skip the frame pointer, function, and
-    // context in the fixed part of the frame.
-    return Operand(rbp, -(index + 3) * kPointerSize);
-  } else {
-    // Incoming parameter. Skip the return address.
-    return Operand(rbp, -(index - 1) * kPointerSize);
-  }
+  return Operand(rbp, StackSlotOffset(op->index()));
 }