[llvm] [DAG] Fold (umin (sub a b) a) -> (usubo a b); (select usubo.1 a usubo.0) (PR #161651)

[Simon Pilgrim via llvm-commits]

================
@@ -6199,6 +6199,19 @@ SDValue DAGCombiner::visitIMINMAX(SDNode *N) {
                                         SDLoc(N), VT, N0, N1))
     return SD;
 
+  // (umin (sub a, b) a) -> (usubo a, b); (select usubo.1, a, usubo.0)
+  {
+    SDValue A, B;
+    if (sd_match(N0, m_Sub(m_Value(A), m_Value(B))) &&
+        sd_match(N1, m_Specific(A)) &&
----------------
RKSimon wrote:

Remove A entirely if its supposed to match N1?
```
if (sd_match(N0, m_Sub(m_Specific(N1), m_Value(B))) &&
```

https://github.com/llvm/llvm-project/pull/161651