[Transforms] Avoid repeated hash lookups (NFC) (#108322)

Author: kazutakahirata

mlir/lib/Transforms/Mem2Reg.cpp

@@ -343,9 +343,8 @@ SmallPtrSet<Block *, 16> MemorySlotPromotionAnalyzer::computeSlotLiveIn(
   // blocks.
   SmallPtrSet<Block *, 16> visited;
   for (Operation *user : slot.ptr.getUsers()) {
-    if (visited.contains(user->getBlock()))
+    if (!visited.insert(user->getBlock()).second)
       continue;
-    visited.insert(user->getBlock());
 
     for (Operation &op : user->getBlock()->getOperations()) {
       if (auto memOp = dyn_cast<PromotableMemOpInterface>(op)) {