[turbofan] cleanup InstructionOperand a little

src/compiler/arm64/code-generator-arm64.cc

Index: src/compiler/arm64/code-generator-arm64.cc
diff --git a/src/compiler/arm64/code-generator-arm64.cc b/src/compiler/arm64/code-generator-arm64.cc
index 762b57a6bc2a8778bdd4f0a3b368e155e1a92953..9298bbaed549b54cdc6ed4127752f46dab4ae9cd 100644
--- a/src/compiler/arm64/code-generator-arm64.cc
+++ b/src/compiler/arm64/code-generator-arm64.cc
@@ -188,7 +188,8 @@ class Arm64OperandConverter FINAL : public InstructionOperandConverter {
     DCHECK(!op->IsDoubleRegister());
     DCHECK(op->IsStackSlot() || op->IsDoubleStackSlot());
     // The linkage computes where all spill slots are located.
-    FrameOffset offset = linkage()->GetFrameOffset(op->index(), frame(), 0);
+    FrameOffset offset = linkage()->GetFrameOffset(
+        AllocatedOperand::cast(op)->index(), frame(), 0);
     return MemOperand(offset.from_stack_pointer() ? masm->StackPointer() : fp,
                       offset.offset());
   }