[CodeGen] Avoid repeated hash lookups (NFC) (#129190)

Author: kazutakahirata

llvm/lib/CodeGen/AssignmentTrackingAnalysis.cpp

@@ -600,12 +600,12 @@ class MemLocFragmentFill {
         break;
     }
 
-    auto CurrentLiveInEntry = LiveIn.find(&BB);
     // If there's no LiveIn entry for the block yet, add it.
-    if (CurrentLiveInEntry == LiveIn.end()) {
+    auto [CurrentLiveInEntry, Inserted] = LiveIn.try_emplace(&BB);
+    if (Inserted) {
       LLVM_DEBUG(dbgs() << "change=true (first) on meet on " << BB.getName()
                         << "\n");
-      LiveIn[&BB] = std::move(BBLiveIn);
+      CurrentLiveInEntry->second = std::move(BBLiveIn);
       return /*Changed=*/true;
     }