[llvm] [Transforms] Copy nsw if it is present on the neg (PR #81496)

[via llvm-commits]

================
@@ -1559,14 +1559,22 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
     bool IntMinIsPoison = cast<Constant>(II->getArgOperand(1))->isOneValue();
 
     // abs(-x) -> abs(x)
-    // TODO: Copy nsw if it was present on the neg?
     Value *X;
-    if (match(IIOperand, m_Neg(m_Value(X))))
-      return replaceOperand(*II, 0, X);
-    if (match(IIOperand, m_Select(m_Value(), m_Value(X), m_Neg(m_Deferred(X)))))
-      return replaceOperand(*II, 0, X);
-    if (match(IIOperand, m_Select(m_Value(), m_Neg(m_Value(X)), m_Deferred(X))))
+    if (match(IIOperand, m_Neg(m_Value(X))) ||
+        match(IIOperand,
+              m_Select(m_Value(), m_Value(X), m_Neg(m_Deferred(X)))) ||
+        match(IIOperand,
+              m_Select(m_Value(), m_Neg(m_Value(X)), m_Deferred(X)))) {
+      // Check if the negation has the nsw flag
+      if (auto *NegInst = dyn_cast<Instruction>(IIOperand)) {
+        if (NegInst->hasNoSignedWrap()) {
----------------
goldsteinn wrote:

This won't work if you match the select case.
Think since you actually need the neg instruction something long the following lines would be better:
```
Value *X, *NegX = nullptr;
if (match(IIOperand, m_Neg(m_Value(X))))
  NegX = IIOperand;
if (match(IIOperand, m_Select(m_Value(), m_Value(X), m_Neg(m_Deferred(X)))))
  NegX = cast<SelectInst>(IIOperand)->FalseVal();
if (match(IIOperand, m_Select(m_Value(), m_Neg(m_Value(X)), m_Deferred(X))))
  NegX = cast<SelectInst>(IIOperand)->TrueVal();
if(NegX != nullptr) {
...
}
```

```

https://github.com/llvm/llvm-project/pull/81496