tint: Optimization: Avoid dynamic cast in TINT_ASSERT

We don't need / want to consider whether the node in the map is of
the SemanticNodeTypeFor type.

Change-Id: Ia0deb3f4e4624bf47e2155fea05f91547d747310
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/96144
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/tint/sem/info.h

@@ -79,7 +79,7 @@ class Info {
     template <typename AST>
     void Add(const AST* ast_node, const SemanticNodeTypeFor<AST>* sem_node) {
         // Check there's no semantic info already existing for the node
-        TINT_ASSERT(Semantic, Get(ast_node) == nullptr);
+        TINT_ASSERT(Semantic, map_.find(ast_node) == map_.end());
         map_.emplace(ast_node, sem_node);
     }