[Transform] Avoid repeated hash lookups (NFC) (#111620)

Author: kazutakahirata

mlir/lib/Dialect/Transform/Transforms/CheckUses.cpp

@@ -39,11 +39,11 @@ template <typename FnTy>
 const llvm::SmallPtrSet<Block *, 4> &
 getReachableImpl(Block *block, FnTy getNextNodes,
                  DenseMap<Block *, llvm::SmallPtrSet<Block *, 4>> &cache) {
-  auto it = cache.find(block);
-  if (it != cache.end())
+  auto [it, inserted] = cache.try_emplace(block);
+  if (!inserted)
     return it->getSecond();
 
-  llvm::SmallPtrSet<Block *, 4> &reachable = cache[block];
+  llvm::SmallPtrSet<Block *, 4> &reachable = it->second;
   SmallVector<Block *> worklist;
   worklist.push_back(block);
   while (!worklist.empty()) {