[llvm] [DAGCombiner] Fold subtraction if above threshold to `umin` (PR #134235)

[Piotr Fusik via llvm-commits]

================
@@ -14895,6 +14895,19 @@ static SDValue performSUBCombine(SDNode *N, SelectionDAG &DAG,
   if (SDValue V = combineSubShiftToOrcB(N, DAG, Subtarget))
     return V;
 
+  if (Subtarget.hasStdExtZbb()) {
+    // fold (sub x, (select (ult x, y), 0, y)) -> (umin x, (sub x, y))
----------------
pfusik wrote:

InstCombine replaces your pattern with mine:

```
define dso_local signext i32 @mod(i32 noundef signext %x, i32 noundef signext %y) local_unnamed_addr #0 {
entry:
  %cmp = icmp uge i32 %x, %y
  %cond = select i1 %cmp, i32 %y, i32 0
  %sub = sub i32 %x, %cond
  ret i32 %sub
}
; *** IR Dump After InstCombinePass on mod ***
; Function Attrs: nounwind uwtable vscale_range(2,1024)
define dso_local signext i32 @mod(i32 noundef signext %x, i32 noundef signext %y) local_unnamed_addr #0 {
entry:
  %cmp.not = icmp ult i32 %x, %y
  %cond = select i1 %cmp.not, i32 0, i32 %y
  %sub = sub i32 %x, %cond
  ret i32 %sub
}
```

https://github.com/llvm/llvm-project/pull/134235