[Turbopack] Dead-code elimination drops code after a `switch`, Server Action returns `undefined` in production

### Link to the code that reproduces this issue https://github.com/albert4719/turbopack-deadcode-repro ### To Reproduce 1. npm install 2. npm run build 3. grep the compiled action chunk in .next/server/chunks/ssr — the code after the switch is gone ### Current vs. Expected behavior A server action with a switch (one empty `case` + break, and `default` returning the same value as another case) loses all code after the switch in `next build`. It returns undefined for the empty case. Works fine in `next dev`. Details + de-minified output in the repo README. ### Provide environment information ```bash Operating System: Platform: win32 Arch: x64 Version: Windows 10 Pro Available memory (MB): 65179 Available CPU cores: 32 Binaries: Node: 26.2.0 npm: 11.13.0 Yarn: 1.22.22 pnpm: N/A Relevant Packages: next: 16.2.9 // Latest available version is detected (16.2.9). eslint-config-next: N/A react: 19.2.4 react-dom: 19.2.4 typescript: N/A Next.js Config: output: N/A ``` ### Which area(s) are affected? (Select all that apply) Turbopack ### Which stage(s) are affected? (Select all that apply) Other (Deployed), next start (local) ### Additional context _No response_