Description:
------------
Heredocs in php generates too many opcodes.

The tokenizer seems to be hooked directly into the zend_do_add_string()  call which creates a new opcode irrespective of previous opcode, where it could do better to just check whether data can be piggy-backed onto the previous opcode. 

Reproduce code:
---------------
<?php 
echo <<<EOF
foo bar is right
EOF;
?>

php52 -dvld.active heredoc.php


Expected result:
----------------
filename:       /tmp/heredoc.php
function name:  (null)
number of ops:  5
line     #  op                           fetch          ext  operands
-------------------------------------------------------------------------------
   4     0  INIT_STRING                                      ~0
         1  ADD_STRING                                       ~0, ~0, 'heredocs+are+fubar'
   5     2  ECHO                                                 ~0
   7     3  RETURN                                               1
         4  ZEND_HANDLE_EXCEPTION        


Actual result:
--------------
filename:       /tmp/heredoc.php
function name:  (null)
number of ops:  10
line     #  op                           fetch          ext  operands
-------------------------------------------------------------------------------
   4     0  INIT_STRING                                      ~0
         1  ADD_STRING                                       ~0, ~0, 'heredocs'
         2  ADD_STRING                                       ~0, ~0, '+'
         3  ADD_STRING                                       ~0, ~0, 'are'
         4  ADD_STRING                                       ~0, ~0, '+'
         5  ADD_STRING                                       ~0, ~0, 'fubar'
   5     6  ADD_STRING                                       ~0, ~0, ''
         7  ECHO                                                 ~0
   7     8  RETURN                                               1
         9  ZEND_HANDLE_EXCEPTION