[turbofan] cleanup InstructionOperand a little

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

Index: src/compiler/ppc/code-generator-ppc.cc
diff --git a/src/compiler/ppc/code-generator-ppc.cc b/src/compiler/ppc/code-generator-ppc.cc
index 44a82c36b621e09b23dc0d4d87391fad495e3dd3..b421f694c8c520190d0250e45e83304e4270ba7a 100644
--- a/src/compiler/ppc/code-generator-ppc.cc
+++ b/src/compiler/ppc/code-generator-ppc.cc
@@ -103,7 +103,8 @@ class PPCOperandConverter 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());
   }
 };