[Transforms] Avoid repeated hash looksup (NFC) (#107727)

Author: kazutakahirata

llvm/lib/Transforms/Utils/LoopUnrollAndJam.cpp

@@ -139,11 +139,9 @@ static bool processHeaderPhiOperands(BasicBlock *Header, BasicBlock *Latch,
   SmallPtrSet<Instruction *, 8> VisitedInstr;
 
   std::function<bool(Instruction * I)> ProcessInstr = [&](Instruction *I) {
-    if (VisitedInstr.count(I))
+    if (!VisitedInstr.insert(I).second)
       return true;
 
-    VisitedInstr.insert(I);
-
     if (AftBlocks.count(I->getParent()))
       for (auto &U : I->operands())
         if (Instruction *II = dyn_cast<Instruction>(U))