fix: variable optimization preserving unused nextContext calls due to listeners

With the recent changes for local refs, we now also generate all
variables in the create block.

This works overall, but we end up with incorrect variable optimization
because calls of `nextContext` in listener functions end up tricking
the optimization logic into thinking that the outer `nextContext` calls
in the create block are _relevant_ for the listener function body.

e.g.

```
if (create) {
  const _bla = nextContext();
  i0.listener(() => {
    restoreView();
    const _ctx1 = nextContext();
    /* do somth with _ctx1 */
  });
}
```

`_bla` can safely be removed here, even though a `ContextRead/Write` has
occurred inside the listener operation. We process listeners separately.

Author: devversion

packages/compiler/src/template/pipeline/src/phases/variable_optimization.ts

@@ -276,10 +276,15 @@ function fencesForIrExpression(expr: ir.Expression): Fence {
 function collectOpInfo(op: ir.CreateOp|ir.UpdateOp): OpInfo {
   let fences = Fence.None;
   const variablesUsed = new Set<ir.XrefId>();
-  ir.visitExpressionsInOp(op, expr => {
+  ir.visitExpressionsInOp(op, (expr, flags) => {
     if (!ir.isIrExpression(expr)) {
       return;
     }
+    // Fences from child operations are not of interest because they allocate
+    // their own variables.
+    if (flags & ir.VisitorContextFlag.InChildOperation) {
+      return;
+    }
 
     switch (expr.kind) {
       case ir.ExpressionKind.ReadVariable: