[llvm] [VectorCombine] Fix transitive Uses in shuffleToVector (PR #188989)

[Simon Pilgrim via llvm-commits]

================
@@ -3701,55 +3702,63 @@ bool VectorCombine::foldShuffleToIdentity(Instruction &I) {
     };
     if (all_of(drop_begin(Item), CheckLaneIsEquivalentToFirst)) {
       // Check the operator is one that we support.
-      if (isa<BinaryOperator, CmpInst>(FrontU)) {
+      if (isa<BinaryOperator, CmpInst>(FrontV)) {
         //  We exclude div/rem in case they hit UB from poison lanes.
-        if (auto *BO = dyn_cast<BinaryOperator>(FrontU);
+        if (auto *BO = dyn_cast<BinaryOperator>(FrontV);
             BO && BO->isIntDivRem())
           return false;
-        Worklist.push_back(generateInstLaneVectorFromOperand(Item, 0));
-        Worklist.push_back(generateInstLaneVectorFromOperand(Item, 1));
+        Worklist.emplace_back(generateInstLaneVectorFromOperand(Item, 0),
+                              &cast<Instruction>(FrontV)->getOperandUse(0));
+        Worklist.emplace_back(generateInstLaneVectorFromOperand(Item, 1),
+                              &cast<Instruction>(FrontV)->getOperandUse(1));
         continue;
       } else if (isa<UnaryOperator, TruncInst, ZExtInst, SExtInst, FPToSIInst,
-                     FPToUIInst, SIToFPInst, UIToFPInst>(FrontU)) {
-        Worklist.push_back(generateInstLaneVectorFromOperand(Item, 0));
+                     FPToUIInst, SIToFPInst, UIToFPInst>(FrontV)) {
+        Worklist.emplace_back(generateInstLaneVectorFromOperand(Item, 0),
+                              &cast<Instruction>(FrontV)->getOperandUse(0));
         continue;
-      } else if (auto *BitCast = dyn_cast<BitCastInst>(FrontU)) {
+      } else if (auto *BitCast = dyn_cast<BitCastInst>(FrontV)) {
         // TODO: Handle vector widening/narrowing bitcasts.
         auto *DstTy = dyn_cast<FixedVectorType>(BitCast->getDestTy());
         auto *SrcTy = dyn_cast<FixedVectorType>(BitCast->getSrcTy());
         if (DstTy && SrcTy &&
             SrcTy->getNumElements() == DstTy->getNumElements()) {
-          Worklist.push_back(generateInstLaneVectorFromOperand(Item, 0));
+          Worklist.emplace_back(generateInstLaneVectorFromOperand(Item, 0),
+                                &cast<Instruction>(FrontV)->getOperandUse(0));
           continue;
         }
-      } else if (isa<SelectInst>(FrontU)) {
-        Worklist.push_back(generateInstLaneVectorFromOperand(Item, 0));
-        Worklist.push_back(generateInstLaneVectorFromOperand(Item, 1));
-        Worklist.push_back(generateInstLaneVectorFromOperand(Item, 2));
+      } else if (isa<SelectInst>(FrontV)) {
----------------
RKSimon wrote:

Pull out casts into isa as a dyn_cast?
```suggestion
      } else if (auto *Sel = dyn_cast<SelectInst>(FrontV)) {
```

https://github.com/llvm/llvm-project/pull/188989