[llvm] [InstCombine] fold `(a == c && b != c) || (a != c && b == c))` to `(a == c) == (b != c)` (PR #94915)

[Nikita Popov via llvm-commits]

================
@@ -3421,6 +3421,32 @@ Value *InstCombinerImpl::foldAndOrOfICmps(ICmpInst *LHS, ICmpInst *RHS,
   return foldAndOrOfICmpsUsingRanges(LHS, RHS, IsAnd);
 }
 
+static Value *foldAorBConst(BinaryOperator &I,
+                            InstCombiner::BuilderTy &Builder) {
+  assert(I.getOpcode() == Instruction::Or &&
+         "Simplification only supports or at the moment.");
+
+  Value *Op0 = I.getOperand(0);
+  Value *Op1 = I.getOperand(1);
+
+  Value *Cmp1, *Cmp2, *Cmp3, *Cmp4;
+  if (!match(Op0, m_And(m_Value(Cmp1), m_Value(Cmp2))) ||
+      !match(Op1, m_And(m_Value(Cmp3), m_Value(Cmp4))))
+    return nullptr;
+
+  // check if any two pairs of the and operations are invertions of each other.
+  Value *Cmp2Inv = nullptr;
+  if (isKnownInversion(Cmp1, Cmp3) && isKnownInversion(Cmp2, Cmp4))
+    Cmp2Inv = Cmp4;
+  if (isKnownInversion(Cmp1, Cmp4) && isKnownInversion(Cmp2, Cmp3))
+    Cmp2Inv = Cmp3;
+
+  if (!Cmp2Inv)
+    return nullptr;
+
+  return Builder.CreateXor(Cmp1, Cmp2Inv);
----------------
nikic wrote:

```suggestion
  if (isKnownInversion(Cmp1, Cmp3) && isKnownInversion(Cmp2, Cmp4))
    return Builder.CreateXor(Cmp1, Cmp4);
  if (isKnownInversion(Cmp1, Cmp4) && isKnownInversion(Cmp2, Cmp3))
    return Builder.CreateXor(Cmp1, Cmp3);
  return nullptr;
```
Would be simpler like this I think...

https://github.com/llvm/llvm-project/pull/94915