[XRay] Simplify code with DenseMap::operator[] (NFC) (#111786)

Author: kazutakahirata

llvm/lib/XRay/BlockIndexer.cpp

@@ -80,12 +80,9 @@ Error BlockIndexer::visit(FunctionRecord &R) {
 }
 
 Error BlockIndexer::flush() {
-  Index::iterator It;
-  std::tie(It, std::ignore) =
-      Indices.insert({{CurrentBlock.ProcessID, CurrentBlock.ThreadID}, {}});
-  It->second.push_back({CurrentBlock.ProcessID, CurrentBlock.ThreadID,
-                        CurrentBlock.WallclockTime,
-                        std::move(CurrentBlock.Records)});
+  Indices[{CurrentBlock.ProcessID, CurrentBlock.ThreadID}].push_back(
+      {CurrentBlock.ProcessID, CurrentBlock.ThreadID,
+       CurrentBlock.WallclockTime, std::move(CurrentBlock.Records)});
   CurrentBlock.ProcessID = 0;
   CurrentBlock.ThreadID = 0;
   CurrentBlock.Records = {};