[llvm] [BDCE] Eliminate unreachable blocks to prevent incorrect replacement (PR #194614)

[Nikita Popov via llvm-commits]

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nikic wrote:

Would it be sufficient to only guard against `BO != BO->getOperand(0)` in this case? That seems to be the only one that could cause self-rauw.

https://github.com/llvm/llvm-project/pull/194614