[clang] [llvm] [Coroutines] Drop dead instructions more aggressively in addMustTailToCoroResumes() (PR #85271)

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zmodem wrote:

> I am wondering if this is a regression from f786881 (this may not be important)

No, it failed before that commit also.

> And could you put the reduced example into the patch in somewhere of `clang/test/CodeGenCoroutines` as a regression test?

Added `coro-symmetric-transfer-04.cpp`.

Please take another look.

https://github.com/llvm/llvm-project/pull/85271