[llvm] [DAGCombiner] Turn `(neg (max x, (neg x)))` into `(min x, (neg x))` (PR #120666)

[Min-Yih Hsu via llvm-commits]

================
@@ -3949,6 +3949,40 @@ SDValue DAGCombiner::visitSUB(SDNode *N) {
       if (SDValue Result = TLI.expandABS(N1.getNode(), DAG, true))
         return Result;
 
+    // Similar to the previous rule, but this time targeting an expanded abs.
+    // (sub 0, (max X, (sub 0, X))) --> (min X, (sub 0, X))
+    // as well as
+    // (sub 0, (min X, (sub 0, X))) --> (max X, (sub 0, X))
+    // Note that these two are applicable to both signed and unsigned min/max.
+    SDValue X;
+    SDValue S0;
+    auto NegPat = m_AllOf(m_Neg(m_Deferred(X)), m_Value(S0));
+    if (LegalOperations &&
----------------
mshockwave wrote:

> I doubt in practice the legality of min is different than max

This, and even if max is illegal, I think it'll be expanded (while min is not), which makes this transformation even more profitable. 

https://github.com/llvm/llvm-project/pull/120666