[llvm] [Coro] Allow spilling @llvm.coro.suspend() to the coro frame (PR #133088)

[Reid Kleckner via llvm-commits]

rnk wrote:

> Since the return value is not constant throughout the coroutine and may get used across suspend points, it must be eligible for spilling like other values, as demonstrated by the bug.

Please add more info to the commit message, here's my draft:

While IR from the frontend typically does not use these suspend return values across suspend points, mid-level optimizations on the presplit coroutine may introduce new uses of suspend values, so they must be considered eligible to spill to the coroutine frame.

https://github.com/llvm/llvm-project/pull/133088