[DeadCode] Add RemoveOverriddenAssignBeforeIfElseRector

[TomasVotruba]

Adds a dead-code rule that removes an assignment placed right before an if/else when both branches fully override the same variable before reading it - so the initial value is never used.

 $result = [];
 if ($value) {
     $result = [1, 2, 3];
 } else {
     $result = [4, 5, 6];
 }

 return $result;

becomes:

 if ($value) {
     $result = [1, 2, 3];
 } else {
     $result = [4, 5, 6];
 }

 return $result;

Guards, to stay safe:

• requires a real else (no elseif chain, no missing else)
• skips when the initial assigned value has a side effect (e.g. a method call)
• skips when the if condition reads the variable
• each branch must fully override the variable in its first statement that touches it - a partial write ($result[] = ...) or a self-read ($result = array_merge($result, ...)) is left alone
• skips static/global/by-ref variables