perf(semantic): remove branch from Nodes::add_node (#4361)

`AstNodes::add_node` had a branch specifically to handle `Program`. Remove that by inlining the special logic for `Program` into `visit_program`.

Probably won't have much effect on benchmarks as the branch is easy to predict, but still removes a few instructions and makes `add_node` easier for compiler to inline.

Author: overlookmotel

crates/oxc_semantic/src/builder.rs

@@ -529,7 +529,19 @@ impl<'a> Visit<'a> for SemanticBuilder<'a> {
         });
         /* cfg - must be above directives as directives are in cfg */
 
-        self.enter_node(kind);
+        // Don't call `enter_node` here as `Program` is a special case - node has no `parent_id`.
+        // Inline the specific logic for `Program` here instead.
+        // This avoids `Nodes::add_node` having to handle the special case.
+        // We can also skip calling `self.enter_kind`, and `self.jsdoc.retrieve_attached_jsdoc`
+        // as they are no-ops for `Program`.
+        self.current_node_id = self.nodes.add_program_node(
+            kind,
+            self.current_scope_id,
+            control_flow!(self, |cfg| cfg.current_node_ix),
+            self.current_node_flags,
+        );
+        self.record_ast_node();
+
         self.enter_scope(
             {
                 let mut flags = ScopeFlags::Top;

crates/oxc_semantic/src/node.rs

@@ -142,6 +142,7 @@ impl<'a> AstNodes<'a> {
     }
 
     /// Create and add an `AstNode` to the `AstNodes` tree and returns its `AstNodeId`.
+    /// Node must not be `Program`. Use `add_program_node` instead.
     pub fn add_node(
         &mut self,
         kind: AstKind<'a>,
@@ -150,14 +151,22 @@ impl<'a> AstNodes<'a> {
         cfg_id: BasicBlockId,
         flags: NodeFlags,
     ) -> AstNodeId {
-        let ast_node_id = match kind {
-            AstKind::Program(_) => {
-                let id = self.parent_ids.push(None);
-                self.root = Some(id);
-                id
-            }
-            _ => self.parent_ids.push(Some(parent_node_id)),
-        };
+        let ast_node_id = self.parent_ids.push(Some(parent_node_id));
+        let node = AstNode::new(kind, scope_id, cfg_id, flags, ast_node_id);
+        self.nodes.push(node);
+        ast_node_id
+    }
+
+    /// Create and add an `AstNode` to the `AstNodes` tree and returns its `AstNodeId`.
+    pub fn add_program_node(
+        &mut self,
+        kind: AstKind<'a>,
+        scope_id: ScopeId,
+        cfg_id: BasicBlockId,
+        flags: NodeFlags,
+    ) -> AstNodeId {
+        let ast_node_id = self.parent_ids.push(None);
+        self.root = Some(ast_node_id);
         let node = AstNode::new(kind, scope_id, cfg_id, flags, ast_node_id);
         self.nodes.push(node);
         ast_node_id