[clang] [llvm] [InstCombine] Infer disjoint flag on Or instructions. (PR #72912)

[Craig Topper via cfe-commits]

topperc wrote:

@nikic is something like this the right fix

```
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 8c29c242215d..b03a56c922de 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -235,8 +235,11 @@ bool llvm::haveNoCommonBitsSet(const WithCache<const Value *> &LHSCache,
          "LHS and RHS should be integers");
 
   if (haveNoCommonBitsSetSpecialCases(LHS, RHS) ||
-      haveNoCommonBitsSetSpecialCases(RHS, LHS))
-    return true;
+      haveNoCommonBitsSetSpecialCases(RHS, LHS)) {
+    if (isGuaranteedNotToBeUndefOrPoison(LHS, SQ.AC, SQ.CxtI, SQ.DT) &&
+        isGuaranteedNotToBeUndefOrPoison(RHS, SQ.AC, SQ.CxtI, SQ.DT))
+      return true;
+  }
 
   return KnownBits::haveNoCommonBitsSet(LHSCache.getKnownBits(SQ),
                                         RHSCache.getKnownBits(SQ));
```

https://github.com/llvm/llvm-project/pull/72912