perf: skip quadratic blocker analysis when no top-level await (#18548)

Skip function reference tracing in `calculate_blockers` when a component
has no top-level `await`.

Blockers only represent dependencies on top-level async statements.
Without a top-level `await`, no binding can have a blocker, so tracing
every top-level function cannot affect the generated output. In large
components with many functions and transitive assignments, that
unnecessary work can become quadratic.

.changeset/fast-cats-compile.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+perf: skip unnecessary blocker analysis when compiling components without top-level await

packages/svelte/src/compiler/phases/2-analyze/index.js

@@ -1221,6 +1221,10 @@ function calculate_blockers(instance, analysis) {
 		}
 	}
 
+	// With no top-level await, no binding can have a blocker and function tracing
+	// cannot affect the output.
+	if (!awaited) return;
+
 	flush_sync_group();
 
 	for (const fn of functions) {