[llvm] [InstCombine] Fold (sub (xor X, (sext C)), (sext C)) => (select C (neg X), X) (PR #79417)

[Simon Pilgrim via llvm-commits]

================
@@ -2448,6 +2448,21 @@ Instruction *InstCombinerImpl::visitSub(BinaryOperator &I) {
     }
   }
 
+  {
+    // (sub (xor X, (sext C)), (sext C)) => (select C, (neg X), X)
+    // (sub (sext C), (xor X, (sext C))) => (select C, X, (neg X))
+    Value *C, *X;
+    auto m_SubXorCmp = [&C, &X](Value *LHS, Value *RHS) {
+      return match(LHS, m_c_Xor(m_Value(X), m_SExt(m_OneUse(m_Value(C))))) &&
----------------
RKSimon wrote:

What is the purpose of the m_OneUse?

Could the patterns be better matched as?
```cpp
return match(LHS, m_c_Xor(m_Value(X), m_Specific(RHS))) &&
       match(RHS, m_SExt(m_Specific(C)) &&
       (C->getType()->getScalarSizeInBits() == 1);
```

https://github.com/llvm/llvm-project/pull/79417