[llvm] [InstCombine] Fold signed-max sub-or pattern (PR #202274)

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mygitljf wrote:

folding to the sign mask is a valid refinement of poison. I'll drop it.I'll update this PR to the generalized form.
Since this generalizes the fold from #202274, which I'm already iterating on after Antonio's review asking for exactly this, I'd prefer to fold the generalization into the original PR to keep the history in one place and avoid two PRs touching the same code.I'm happy to credit your test cases there. Thanks for the help!



https://github.com/llvm/llvm-project/pull/202274