[llvm] [DAGCombiner] Fold vector subtraction if above threshold to `umin` (PR #148834)

[Piotr Fusik via llvm-commits]

================
@@ -4093,6 +4093,26 @@ SDValue DAGCombiner::visitSUB(SDNode *N) {
       return N0;
   }
 
+  // (sub x, ([v]select (ult x, y), 0, y)) -> (umin x, (sub x, y))
+  // (sub x, ([v]select (uge x, y), y, 0)) -> (umin x, (sub x, y))
+  if (N1.hasOneUse() && hasUMin(VT)) {
+    SDValue Y;
+    if (sd_match(N1, m_Select(m_SetCC(m_Specific(N0), m_Value(Y),
+                                      m_SpecificCondCode(ISD::SETULT)),
+                              m_Zero(), m_Deferred(Y))) ||
+        sd_match(N1, m_Select(m_SetCC(m_Specific(N0), m_Value(Y),
+                                      m_SpecificCondCode(ISD::SETUGE)),
+                              m_Deferred(Y), m_Zero())) ||
+        sd_match(N1, m_VSelect(m_SetCC(m_Specific(N0), m_Value(Y),
+                                       m_SpecificCondCode(ISD::SETULT)),
+                               m_Zero(), m_Deferred(Y))) ||
+        sd_match(N1, m_VSelect(m_SetCC(m_Specific(N0), m_Value(Y),
+                                       m_SpecificCondCode(ISD::SETUGE)),
+                               m_Deferred(Y), m_Zero())))
+      return DAG.getNode(ISD::UMIN, DL, VT, N0,
+                         DAG.getNode(ISD::SUB, DL, VT, N0, Y));
+  }
+
----------------
pfusik wrote:

Moved before:
```cpp
  if (SDValue NewSel = foldBinOpIntoSelect(N))
```
which transforms this sub-maybe-zero pattern to a conditional sub in `foldSelectWithIdentityConstant`.


https://github.com/llvm/llvm-project/pull/148834