perf(compiler-cli): only generate template context declaration when used (#39321)

The variable declaration for a template context is only needed when it
is referenced from somewhere, so the TCB operation to generate the
declaration is marked as optional.

PR Close #39321

Author: JoostK

packages/compiler-cli/src/ngtsc/typecheck/src/type_check_block.ts

@@ -194,9 +194,8 @@ class TcbTemplateContextOp extends TcbOp {
     super();
   }
 
-  get optional() {
-    return false;
-  }
+  // The declaration of the context variable is only needed when the context is actually referenced.
+  readonly optional = true;
 
   execute(): ts.Identifier {
     // Allocate a template ctx variable and declare it with an 'any' type. The type of this variable

packages/compiler-cli/src/ngtsc/typecheck/test/type_check_block_spec.ts

@@ -763,13 +763,13 @@ describe('type check blocks', () => {
 
       it('generates a references var when enabled', () => {
         const block = tcb(TEMPLATE, DIRECTIVES);
-        expect(block).toContain('var _t2 = (_t1 as any as core.TemplateRef<any>);');
+        expect(block).toContain('var _t1 = (_t2 as any as core.TemplateRef<any>);');
       });
 
       it('generates a reference var when disabled', () => {
         const DISABLED_CONFIG: TypeCheckingConfig = {...BASE_CONFIG, checkTemplateBodies: false};
         const block = tcb(TEMPLATE, DIRECTIVES, DISABLED_CONFIG);
-        expect(block).toContain('var _t2 = (_t1 as any as core.TemplateRef<any>);');
+        expect(block).toContain('var _t1 = (_t2 as any as core.TemplateRef<any>);');
       });
     });
 
@@ -913,8 +913,8 @@ describe('type check blocks', () => {
       it('should trace references to an <ng-template> when enabled', () => {
         const block = tcb(TEMPLATE, DIRECTIVES);
         expect(block).toContain(
-            'var _t4 = (_t3 as any as core.TemplateRef<any>); ' +
-            '"" + (((_t4).value2));');
+            'var _t3 = (_t4 as any as core.TemplateRef<any>); ' +
+            '"" + (((_t3).value2));');
       });
 
       it('should use any for reference types when disabled', () => {