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

[Luke Lau via llvm-commits]

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@@ -4251,6 +4251,19 @@ SDValue DAGCombiner::visitSUB(SDNode *N) {
       sd_match(N1, m_UMaxLike(m_Specific(A), m_Specific(B))))
     return DAG.getNegative(DAG.getNode(ISD::ABDU, DL, VT, A, B), DL, VT);
 
+  // (sub x, (select (ult x, y), 0, y)) -> (umin x, (sub x, y))
+  auto LK = TLI.getTypeConversion(*DAG.getContext(), VT);
+  if ((LK.first == TargetLoweringBase::TypeLegal ||
+       LK.first == TargetLoweringBase::TypePromoteInteger) &&
+      TLI.isOperationLegal(ISD::UMIN, LK.second)) {
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lukel97 wrote:

Does this work if you just do 
```suggestion
  // Allow types that will eventually be made legal
  if (TLI.isOperationLegal(ISD::UMIN, TLI.getTypeToTransformTo(*DAG.getContext(), VT)) {
```

I feel like TLI.getTypeToTransformTo is more common in DAGCombiner than TLI.getTypeConversion. Or does that regress the i64 on rv32 tests which are expanded?

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