[llvm] [InstSimpify] Simplifying `(xor (sub C_Mask, X), C_Mask)` -> `X` (PR #122552)

[Nikita Popov via llvm-commits]

================
@@ -871,6 +871,14 @@ static Value *simplifySubInst(Value *Op0, Value *Op1, bool IsNSW, bool IsNUW,
   if (Value *V = simplifyByDomEq(Instruction::Sub, Op0, Op1, Q, MaxRecurse))
     return V;
 
+  // (sub nuw C_Mask, (xor X, C_Mask)) -> X
+  if (IsNUW) {
+    Value *X;
+    if (match(Op1, m_Xor(m_Value(X), m_Specific(Op0))) &&
+        match(Op0, m_CheckedInt([](const APInt &C) { return C.isMask(); })))
----------------
nikic wrote:

```suggestion
        match(Op0, m_LowBitMask())
```

https://github.com/llvm/llvm-project/pull/122552