Make the Optimizer Trace of reqular query and PS EXECUTE be identical

Print this piece when we've just made the choice to convert to semi-join.
Also, print it when we've already made that choice before:

  transformation": {
     "select_id": 2,
     "from": "IN (SELECT)",
     "to": "semijoin",
     "chosen": true
   }

mysql-test/main/opt_trace,ps.rdiff

---- /Users/shulga/projects/mariadb/server-10.6/mysql-test/main/opt_trace.result	2021-07-21 19:17:11.000000000 +0700
-+++ /Users/shulga/projects/mariadb/server-10.6/mysql-test/main/opt_trace.reject	2021-07-21 19:17:48.000000000 +0700
-@@ -2829,14 +2829,6 @@
-                   }
-                 },
-                 {
--                  "transformation": {
--                    "select_id": 2,
--                    "from": "IN (SELECT)",
--                    "to": "semijoin",
--                    "chosen": true
--                  }
--                },
--                {
-                   "expanded_query": "/* select#2 */ select t10.pk from t10"
-                 }
-               ]
-@@ -4402,14 +4394,6 @@
-                   }
-                 },
-                 {
--                  "transformation": {
--                    "select_id": 2,
--                    "from": "IN (SELECT)",
--                    "to": "semijoin",
--                    "chosen": true
--                  }
--                },
--                {
-                   "expanded_query": "/* select#2 */ select t_inner_1.a from t1 t_inner_1 join t1 t_inner_2"
-                 }
-               ]
-@@ -4852,14 +4836,6 @@
-                   }
-                 },
-                 {
--                  "transformation": {
--                    "select_id": 2,
--                    "from": "IN (SELECT)",
--                    "to": "semijoin",
--                    "chosen": true
--                  }
--                },
--                {
-                   "expanded_query": "/* select#2 */ select t_inner_1.a from t2 t_inner_2 join t1 t_inner_1"
-                 }
-               ]
-@@ -4879,14 +4855,6 @@
-                   }
-                 },
-                 {
--                  "transformation": {
--                    "select_id": 3,
--                    "from": "IN (SELECT)",
--                    "to": "semijoin",
--                    "chosen": true
--                  }
--                },
--                {
-                   "expanded_query": "/* select#3 */ select t_inner_3.a from t2 t_inner_3 join t1 t_inner_4"
-                 }
-               ]
-@@ -6432,14 +6400,6 @@
-                   }
-                 },
-                 {
--                  "transformation": {
--                    "select_id": 2,
--                    "from": "IN (SELECT)",
--                    "to": "semijoin",
--                    "chosen": true
--                  }
--                },
--                {
-                   "expanded_query": "/* select#2 */ select t_inner_1.a from t2 t_inner_2 join t1 t_inner_1"
-                 }
-               ]
-@@ -6459,14 +6419,6 @@
-                   }
-                 },
-                 {
--                  "transformation": {
--                    "select_id": 3,
--                    "from": "IN (SELECT)",
--                    "to": "semijoin",
--                    "chosen": true
--                  }
--                },
--                {
-                   "expanded_query": "/* select#3 */ select t_inner_3.a from t2 t_inner_3 join t1 t_inner_4"
-                 }
-               ]

mysql-test/main/opt_trace.test

 --source include/not_embedded.inc
 --source include/have_sequence.inc
---source include/protocol.inc
 SELECT table_name, column_name FROM information_schema.columns where table_name="OPTIMIZER_TRACE";
 set optimizer_trace="enabled=on";
 show variables like 'optimizer_trace';

sql/opt_subselect.cc

@@ -717,6 +717,15 @@ int check_and_do_in_subquery_rewrites(JOIN *join)
         if (arena)
           thd->restore_active_arena(arena, &backup);
         in_subs->is_registered_semijoin= TRUE;
+      }
+
+      /*
+        Print the transformation into trace. Do it when we've just set
+        is_registered_semijoin=TRUE above, and also do it when we've already
+        had it set.
+      */
+      if (in_subs->is_registered_semijoin)
+      {
         OPT_TRACE_TRANSFORM(thd, trace_wrapper, trace_transform,
                             select_lex->select_number,
                             "IN (SELECT)", "semijoin");