Mark VtablePtr always constant (#5843)

toolchain/lower/handle.cpp

@@ -300,9 +300,4 @@ auto HandleInst(FunctionContext& context, SemIR::InstId inst_id,
                    context.CreateAlloca(context.GetTypeOfInst(inst_id)));
 }
 
-auto HandleInst(FunctionContext& /*context*/, SemIR::InstId /*inst_id*/,
-                SemIR::VtablePtr /*inst*/) -> void {
-  CARBON_FATAL("`VtablePtr` insts should always be constant");
-}
-
 }  // namespace Carbon::Lower

toolchain/sem_ir/typed_insts.h

@@ -1779,8 +1779,7 @@ struct VtableType {
 // Initializer for virtual function table pointers in object initialization.
 struct VtablePtr {
   static constexpr auto Kind = InstKind::VtablePtr.Define<Parse::NodeId>(
-      {.ir_name = "vtable_ptr",
-       .constant_kind = InstConstantKind::WheneverPossible});
+      {.ir_name = "vtable_ptr", .constant_kind = InstConstantKind::Always});
   TypeId type_id;
   VtableId vtable_id;
   SpecificId specific_id;