[llvm] [llvm][Coro] Lower coro.frame in CoroEarly instead of CoroSplit (PR #137976)

[Weibo He via llvm-commits]

NewSigma wrote:

> What's the benefit except aligning the document?

This is a refactor to improve readability and code quality. Ideally, `coro::Shape` should behave as an analyzer and should not contain any transformations. `coro.frame` is a helper intrinsic and is not used by any middle-end passes. I think it would be good to lower it as early as possible.

https://github.com/llvm/llvm-project/pull/137976