[llvm] [Coroutines] Implement elision for noop coroutines (PR #174353)

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bgra8 wrote:

Heads-up @NewSigma  we (at google) have bisected something that looks like a miscompile to this revision.

I'm working on reducing the test case to a small reproducer.

https://github.com/llvm/llvm-project/pull/174353