[llvm] [DAGCombiner] Don't fold cheap extracts of multiple use splats (PR #134120)

[Simon Pilgrim via llvm-commits]

================
@@ -25383,8 +25383,10 @@ SDValue DAGCombiner::visitEXTRACT_SUBVECTOR(SDNode *N) {
   // ty1 extract_vector(ty2 splat(V))) -> ty1 splat(V)
   if (V.getOpcode() == ISD::SPLAT_VECTOR)
     if (DAG.isConstantValueOfAnyType(V.getOperand(0)) || V.hasOneUse())
-      if (!LegalOperations || TLI.isOperationLegal(ISD::SPLAT_VECTOR, NVT))
-        return DAG.getSplatVector(NVT, DL, V.getOperand(0));
+      if (!TLI.isExtractSubvectorCheap(NVT, V.getValueType(), ExtIdx) ||
+          V.hasOneUse())
----------------
RKSimon wrote:

merge these clauses?
```cpp
if ((DAG.isConstantValueOfAnyType(V.getOperand(0)) && !TLI.isExtractSubvectorCheap(NVT, V.getValueType(), ExtIdx) || V.hasOneUse())
```

https://github.com/llvm/llvm-project/pull/134120