[llvm] [InstCombine] Convert load from LUT into a select (PR #98339)

[via llvm-commits]

erikdesjardins wrote:

> I am not willing to improve this fold for undef/poison as it is unlikely to have real-world usefulness.

I agree. What I mean is--disabling undef in this fold does not really fix the issue, if the root cause is elsewhere. Presumably the same miscompile could still happen, with the right input IR.

(Perhaps it doesn't matter so much if we're planning to remove undef anyways.)

https://github.com/llvm/llvm-project/pull/98339