Auto merge of #40367 - eddyb:naked-cruft, r=nagisa

Improve the LLVM IR we generate for trivial functions, especially #[naked] ones.

These two small changes fix edef1c/libfringe#68:
* Don't emit ZST allocas, such as when returning `()`
* Don't emit a branch from LLVM's entry block to MIR's `START_BLOCK` unless needed
  * That is, if a loop branches back to it, although I'm not sure that's even valid MIR