PS related C++ code blocks diverged in the two *.yy files.
Under terms of this task we'll synchronize the code.
Let's move these LEX members:

  LEX_CSTRING prepared_stmt_name;

  Item *prepared_stmt_code;

  List<Item> prepared_stmt_params;

into a separate new class Lex_prepared_stmt, move the C++ code blocks to LEX and to the new class as methods and reuse these methods in *.yy files.
In the new reduction the grammar will look as simple as follows:

prepare:

          PREPARE_SYM ident FROM prepare_src

          {

            if (Lex->stmt_prepare($2, $4))

              MYSQL_YYABORT;

          }

        ;

execute:

          EXECUTE_SYM ident execute_using

          {

            if (Lex->stmt_execute($2, $3))

              MYSQL_YYABORT;

          }

        | EXECUTE_SYM IMMEDIATE_SYM prepare_src execute_using

          {

            if (Lex->stmt_execute_immediate($3, $4))

              MYSQL_YYABORT;

          }

        ;

i.e. one method call per statement.

Note, a separate class Lex_prepared_stmt will help to move PS related data structures from LEX to new new classes for all PS based statements:

• Sql_cmd_prepare
• Sql_cmd_deallocate
• Sql_cmd_execute
• Sql_cmd_execute_immediate

This will be done separately, eventually.