Use elseif for mutually exclusive constant array check

The two branches in setExistingOffsetValueType() are mutually exclusive
(first checks isConstantArray()->yes(), second checks !isConstantArray()->yes()),
so use elseif to make this explicit and save unnecessary condition evaluation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: phpstan-bot

src/Type/ArrayType.php

@@ -408,9 +408,7 @@ public function setExistingOffsetValueType(Type $offsetType, Type $valueType): T
 					$newItemType,
 				);
 			}
-		}
-
-		if (
+		} elseif (
 			$this->itemType->isArray()->yes()
 			&& !$this->itemType->isConstantArray()->yes()
 			&& $valueType->isArray()->yes()