[turbofan] cleanup InstructionOperand a little

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

Index: src/compiler/arm/code-generator-arm.cc
diff --git a/src/compiler/arm/code-generator-arm.cc b/src/compiler/arm/code-generator-arm.cc
index 6ea5e1bde2c22347b16eeeededd3d657f163cc46..e9a7c9f380770c2935a1af73adffb962ab3d714c 100644
--- a/src/compiler/arm/code-generator-arm.cc
+++ b/src/compiler/arm/code-generator-arm.cc
@@ -151,7 +151,8 @@ class ArmOperandConverter 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() ? sp : fp, offset.offset());
   }
 };