[CodeQuality] Skip SimplifyUselessVariableRector on commented-out code and inline concat

[TomasVotruba]

Two changes to SimplifyUselessVariableRector:

1. Skip when commented-out code between the assign and the return mentions the variable. Such code is often re-enabled later, so inlining the assign into the return removes the variable it relies on.

$pdf = curl_exec($ch);

// $url = ...;
// $pdf = curl_exec($ch);
// curl_close($ch);

return $pdf;

2. Stop merging compound assigns (.=, +=, ...) into the return. Folding $xml .= '</RuleMessage>'; return $xml; into return $xml . '</RuleMessage>'; worsens readability. Only direct assigns are simplified now, so the only_direct_assign option and its non-direct branch are removed.