Optimized away excessive condition

trx_set_rw_mode() is never called for read-only transactions, this is guarded
by callers.

Removing this condition from critical section immediately gives 5% scalability
improvement in OLTP index updates benchmark.

storage/innobase/trx/trx0trx.cc

@@ -2957,6 +2957,7 @@ trx_set_rw_mode(
 	ut_ad(trx->rsegs.m_redo.rseg == 0);
 	ut_ad(!trx->in_rw_trx_list);
 	ut_ad(!trx_is_autocommit_non_locking(trx));
+	ut_ad(!trx->read_only);
 
 	if (high_level_read_only) {
 		return;
@@ -2993,11 +2994,9 @@ trx_set_rw_mode(
 	}
 #endif /* UNIV_DEBUG */
 
-	if (!trx->read_only) {
-		UT_LIST_ADD_FIRST(trx_sys->rw_trx_list, trx);
+	UT_LIST_ADD_FIRST(trx_sys->rw_trx_list, trx);
 
-		ut_d(trx->in_rw_trx_list = true);
-	}
+	ut_d(trx->in_rw_trx_list = true);
 
 	mutex_exit(&trx_sys->mutex);
 }