[llvm] [TailRecElim] Handle discarded-call return conflicts with a shift accumulator (PR #221694)

[Federico Bruzzone via llvm-commits]

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FedericoBruzzone wrote:

I just copy-pasted the example from the @nikic suggestion. See https://github.com/llvm/llvm-project/pull/181331#issuecomment-5509668449.

However, I renamed the BB to `discard`, since it's the call site whose recursive-call result is discarded :'D

https://github.com/llvm/llvm-project/pull/221694