Rename isSimpleVariableReference() to isVariableToVariableReference() for clarity

The new name better describes what the method checks: whether both sides
of the intertwined reference link are plain named variables.

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

Author: phpstan-bot

src/Analyser/MutatingScope.php

@@ -2810,7 +2810,7 @@ public function invalidateExpression(Expr $expressionToInvalidate, bool $require
 			$exprExpr = $exprTypeHolder->getExpr();
 			if (
 				$exprExpr instanceof IntertwinedVariableByReferenceWithExpr
-				&& $exprExpr->isSimpleVariableReference()
+				&& $exprExpr->isVariableToVariableReference()
 			) {
 				continue;
 			}

src/Node/Expr/IntertwinedVariableByReferenceWithExpr.php

@@ -31,7 +31,7 @@ public function getAssignedExpr(): Expr
 		return $this->assignedExpr;
 	}
 
-	public function isSimpleVariableReference(): bool
+	public function isVariableToVariableReference(): bool
 	{
 		return $this->expr instanceof Variable
 			&& is_string($this->expr->name)