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/sql_cache.cc

@@ -1052,7 +1052,7 @@ void query_cache_insert(void *thd_arg, const char *packet, size_t length,
     called for this thread.
   */
 
-  if (!thd)
+  if (unlikely(!thd))
     return;
 
   query_cache.insert(thd, &thd->query_cache_tls,