[llvm] [InstCombine] fold `(icmp eq/ne (or disjoint x, C0), C1)` -> `(icmp eq/ne x, C0^C1)` (PR #87734)

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goldsteinn wrote:

> Given how the test cases show variables and not just constants, can this fold apply to variables too?

I don't think the fold is profitable if we can't constant fold the `xor`.

https://github.com/llvm/llvm-project/pull/87734