[llvm] [InstCombine] Fold `select Cond, not X, X` into `Cond ^ X` (PR #90089)

[via llvm-commits]

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@@ -3985,5 +3985,10 @@ Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
     }
   }
 
+  if (CondVal->getType() == SI.getType() && impliesPoison(FalseVal, TrueVal) &&
+      isImpliedCondition(FalseVal, TrueVal, DL, /*LHSIsTrue=*/true) == false &&
+      isImpliedCondition(FalseVal, TrueVal, DL, /*LHSIsTrue=*/false) == true)
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goldsteinn wrote:

Maybe add a comment that the inverse case doesn't naturally occur in code to avoid people re-testing / re-submitting.

https://github.com/llvm/llvm-project/pull/90089