Add likely/unlikely to speed up execution

Added to:
- if (error)
- Lex
- sql_yacc.yy and sql_yacc_ora.yy
- In header files to alloc() calls
- Added thd argument to thd_net_is_killed()

sql/threadpool_common.cc

@@ -171,7 +171,7 @@ void tp_callback(TP_connection *c)
 
   c->state = TP_STATE_RUNNING;
 
-  if (!thd)
+  if (unlikely(!thd))
   {
     /* No THD, need to login first. */
     DBUG_ASSERT(c->connect);