[llvm] [CoroEarly] Hide promise alloca for later passes (PR #139243)

[Chuanqi Xu via llvm-commits]

ChuanqiXu9 wrote:

> > I think it is better to consider if it should be instead of the numbers. coro.promise() is not appropriate since it is a consumer. We need a producer for this.
> 
> Well, I don’t see a strong reason not to use `coro.promise`. After all, the promise is collected into the coro frame, and essentially, it’s the frame that produces the coro promise. We still need a consumer (GEP) to locate the promise. But if you think this isn’t the right direction to address the issue, I can try using another intrinsic—maybe `coro.promise.alloca`? I imagine that would be a larger project and might involve more code and documentation changes.

It is a thought, not a requirement. If your experiment with `coro.promise` succeed, it should be good too.

https://github.com/llvm/llvm-project/pull/139243