[PDL] Avoid repeated hash lookups (NFC) (#108927)

Author: kazutakahirata

mlir/lib/Dialect/PDL/IR/PDL.cpp

@@ -65,13 +65,10 @@ static void visit(Operation *op, DenseSet<Operation *> &visited) {
   if (!isa<PatternOp>(op->getParentOp()) || isa<RewriteOp>(op))
     return;
 
-  // Ignore if already visited.
-  if (visited.contains(op))
+  // Ignore if already visited.  Otherwise, mark as visited.
+  if (!visited.insert(op).second)
     return;
 
-  // Mark as visited.
-  visited.insert(op);
-
   // Traverse the operands / parent.
   TypeSwitch<Operation *>(op)
       .Case<OperationOp>([&visited](auto operation) {