Optimiser logic addition. Deletion of unused variables

[NickSettler]

It is needed to add deletion of unused variables to code optimiser. Variable can be deleted in the following cases:

• It is not used anywhere in the code
• It is used only in assignment expression, where the left part of assignment is a variable going to be deleted

// First case example
$a = 1;         # this line can be deleted
write("Hello world");

// Second case example
$i = 0;
$b = 0;         # this line can be deleted
while ($i < 10) {
  $b = $b + 1;  # this line can be deleted
  $i = $i + 1;
}
write($i);