refactor(compiler): pipeline phase to merge sequential elementstart/elementend (#48580)

As an optimization, the runtime provides an `element()` instruction to use in
place of `elementStart()` and `elementEnd()` when there are no instructions
between them. This commit merges `ElementStart` and `ElementEnd` IR operations
into `Element` operations in that special case.

PR Close #48580

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

@@ -0,0 +1,30 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import * as ir from '../../ir';
+import {ComponentCompilation} from '../compilation';
+
+/**
+ * Replace sequences of `ElementStart` followed by `ElementEnd` with a condensed `Element`
+ * instruction.
+ */
+export function phaseEmptyElements(cpl: ComponentCompilation): void {
+  for (const [_, view] of cpl.views) {
+    for (const op of view.create) {
+      if (op.kind === ir.OpKind.ElementEnd && op.prev !== null &&
+          op.prev.kind === ir.OpKind.ElementStart) {
+        // Transmute the `ElementStart` instruction to `Element`. This is safe as they're designed
+        // to be identical apart from the `kind`.
+        (op.prev as unknown as ir.ElementOp).kind = ir.OpKind.Element;
+
+        // Remove the `ElementEnd` instruction.
+        ir.OpList.remove<ir.CreateOp>(op);
+      }
+    }
+  }
+}