When replacing WHERE f with WHERE f IS TRUE mark it AT_TOP_LEVEL

fixes innodb_fts.fulltext_misc failures after MDEV-37653

also remove redundant `if ($3)` - it can never be NULL
and fix a function comment.

Author: vuvova

sql/sql_parse.cc

@@ -8921,8 +8921,8 @@ push_new_name_resolution_context(THD *thd,
 
 
 /**
-  Fix condition which contains only field (f turns to  f <> 0 )
-    or only contains the function NOT field (not f turns to  f == 0)
+  Fix condition which contains only field (f turns to  f IS TRUE )
+  or only contains the function NOT field (not f turns to  f IS FALSE)
 
   @param cond            The condition to fix
 
@@ -8936,7 +8936,8 @@ Item *normalize_cond(THD *thd, Item *cond)
     Item::Type type= cond->type();
     if (type == Item::FIELD_ITEM || type == Item::REF_ITEM)
     {
-      item_base_t is_cond_flag= cond->base_flags & item_base_t::IS_COND;
+      item_base_t is_cond_flag= cond->base_flags &
+        (item_base_t::IS_COND | item_base_t::AT_TOP_LEVEL);
       cond->base_flags&= ~item_base_t::IS_COND;
       cond= new (thd->mem_root) Item_func_istrue(thd, cond);
       if (cond)

sql/sql_yacc.yy

@@ -12457,11 +12457,9 @@ opt_where_clause:
           }
           search_condition
           {
-            SELECT_LEX *select= Select;
-            select->where= normalize_cond(thd, $3);
-            select->parsing_place= NO_MATTER;
-            if ($3)
-              $3->top_level_item();
+            $3->top_level_item();
+            Select->where= normalize_cond(thd, $3);
+            Select->parsing_place= NO_MATTER;
           }
         ;