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/thr_malloc.cc

@@ -27,7 +27,7 @@ extern "C" {
   void sql_alloc_error_handler(void)
   {
     THD *thd= current_thd;
-    if (thd)
+    if (likely(thd))
     {
       if (! thd->is_error())
       {