Add test for changing the name of an existing pass when .registering it.

t/tree-optimizer.t

@@ -2,7 +2,7 @@
 
 pir::load_bytecode('Tree/Optimizer.pbc');
 
-plan(32);
+plan(33);
 
 {
     my $opt := Tree::Optimizer.new;
@@ -307,6 +307,14 @@ pir::load_bytecode('PAST/Pattern.pbc');
     }
 }
 
+{
+    my $inc := Tree::Optimizer::Pass.new(-> $n { $n + 1; });
+    my $opt := Tree::Optimizer.new;
+    $opt.register($inc, :name<inc>);
+    ok(pir::defined__IP($opt.find-pass('inc')),
+       'Adding .name to an existing pass object when .registering works.');
+}
+
 {
     my $inc := Tree::Optimizer::Pass.new(-> $n { $n + 1; }, :name<inc>);
     my $double := Tree::Optimizer::Pass.new(-> $n { $n * 2; }, :name<double>);