[turbofan] cleanup InstructionOperand a little

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

Index: src/compiler/mips/code-generator-mips.cc
diff --git a/src/compiler/mips/code-generator-mips.cc b/src/compiler/mips/code-generator-mips.cc
index ec545156e55704b2d5701b2c56ace6baaa6376ff..1dee98b9f425c35cf660277a488e772c8de1a18b 100644
--- a/src/compiler/mips/code-generator-mips.cc
+++ b/src/compiler/mips/code-generator-mips.cc
@@ -110,7 +110,8 @@ class MipsOperandConverter 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());
   }
 };