Ignore false-positive cheaper-guard order in DowngradeSubstrFalsyRector (#391)

The isName() guard cannot move above the CallLike block, which marks
IS_FALSY_UNCASTABLE on arguments of non-substr call nodes; hoisting would skip
that side effect and wrongly rewrite nested substr() in falsy-tolerant params.

Claude-Session: https://claude.ai/code/session_01Na4eYvNqa2pdhZrBXbegrv

Author: TomasVotruba

rules/DowngradePhp80/Rector/FuncCall/DowngradeSubstrFalsyRector.php

@@ -178,6 +178,7 @@ public function refactor(Node $node): ?Node
             }
         }
 
+        // @phpstan-ignore rector.rectorCheaperGuardsFirst (cannot hoist above the CallLike block, which marks IS_FALSY_UNCASTABLE on non-substr nodes)
         if (! $this->isName($node, 'substr')) {
             return null;
         }