[SLP] Avoid repeated visitation in getVectorElementSize(); NFC

We need to insert into the Visited set at the same time we insert
into the worklist. Otherwise we may end up pushing the same
instruction to the worklist multiple times, and only adding it to
the visited set later.

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -5308,16 +5308,17 @@ unsigned BoUpSLP::getVectorElementSize(Value *V) const {
   // of memory operations where possible.
   SmallVector<Instruction *, 16> Worklist;
   SmallPtrSet<Instruction *, 16> Visited;
-  if (auto *I = dyn_cast<Instruction>(V))
+  if (auto *I = dyn_cast<Instruction>(V)) {
     Worklist.push_back(I);
+    Visited.insert(I);
+  }
 
   // Traverse the expression tree in bottom-up order looking for loads. If we
   // encounter an instruction we don't yet handle, we give up.
   auto MaxWidth = 0u;
   auto FoundUnknownInst = false;
   while (!Worklist.empty() && !FoundUnknownInst) {
     auto *I = Worklist.pop_back_val();
-    Visited.insert(I);
 
     // We should only be looking at scalar instructions here. If the current
     // instruction has a vector type, give up.
@@ -5337,7 +5338,7 @@ unsigned BoUpSLP::getVectorElementSize(Value *V) const {
              isa<CmpInst>(I) || isa<SelectInst>(I) || isa<BinaryOperator>(I)) {
       for (Use &U : I->operands())
         if (auto *J = dyn_cast<Instruction>(U.get()))
-          if (!Visited.count(J))
+          if (Visited.insert(J).second)
             Worklist.push_back(J);
     }