[llvm] [InstCombine] Resolve TODO: Remove one-time check if other logic operand (Y) is constant (PR #77973)

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AtariDreams wrote:

@nikic Alright. I think I got this right. Though I am concerned about the shl undef tests that seem to fold things into poison values. I do not know if this is what we want.

https://github.com/llvm/llvm-project/pull/77973