[turbofan] cleanup InstructionOperand a little

src/compiler/x64/code-generator-x64.cc

Index: src/compiler/x64/code-generator-x64.cc
diff --git a/src/compiler/x64/code-generator-x64.cc b/src/compiler/x64/code-generator-x64.cc
index 443f1b1efdd4676d601da77cc02c2f5e0971cc6a..a60b489e42d91f6c7bd6aee6f264b66ac64c9ae9 100644
--- a/src/compiler/x64/code-generator-x64.cc
+++ b/src/compiler/x64/code-generator-x64.cc
@@ -44,7 +44,8 @@ class X64OperandConverter : public InstructionOperandConverter {
   Operand ToOperand(InstructionOperand* op, int extra = 0) {
     DCHECK(op->IsStackSlot() || op->IsDoubleStackSlot());
     // The linkage computes where all spill slots are located.
-    FrameOffset offset = linkage()->GetFrameOffset(op->index(), frame(), extra);
+    FrameOffset offset = linkage()->GetFrameOffset(
+        AllocatedOperand::cast(op)->index(), frame(), extra);
     return Operand(offset.from_stack_pointer() ? rsp : rbp, offset.offset());
   }