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

[Chuanqi Xu via llvm-commits]

https://github.com/ChuanqiXu9 commented:

The high level idea looks good. But why do we hack in CoroEarly pass? Can't we generate yet another intrinsic in the frontend? And also, why promise is different from other allocas in the purpose (avoid mis-optimization)?

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