[Transforms] Avoid repeated hash lookups (NFC)

Author: kazutakahirata

llvm/include/llvm/Transforms/Utils/SampleProfileLoaderBaseImpl.h

@@ -646,14 +646,13 @@ void SampleProfileLoaderBaseImpl<BT>::findEquivalenceClasses(FunctionT &F) {
     BasicBlockT *BB1 = &BB;
 
     // Compute BB1's equivalence class once.
-    if (EquivalenceClass.count(BB1)) {
+    // By default, blocks are in their own equivalence class.
+    auto [It, Inserted] = EquivalenceClass.try_emplace(BB1, BB1);
+    if (!Inserted) {
       LLVM_DEBUG(printBlockEquivalence(dbgs(), BB1));
       continue;
     }
 
-    // By default, blocks are in their own equivalence class.
-    EquivalenceClass[BB1] = BB1;
-
     // Traverse all the blocks dominated by BB1. We are looking for
     // every basic block BB2 such that:
     //