Undo change in the handling of 'L->top' (commit b80077b8f3)

With MMBIN instructions, there are fewer opcodes that need to update
'L->top', so that change does not seem to pay for the increased
complexity.

lobject.c

@@ -127,9 +127,7 @@ void luaO_arith (lua_State *L, int op, const TValue *p1, const TValue *p2,
                  StkId res) {
   if (!luaO_rawarith(L, op, p1, p2, s2v(res))) {
     /* could not perform raw operation; try metamethod */
-    ptrdiff_t top = savestack(L, L->top);
     luaT_trybinTM(L, p1, p2, res, cast(TMS, (op - LUA_OPADD) + TM_ADD));
-    L->top = restorestack(L, top);
   }
 }