[clang] [llvm] [mlir] [ADT] Give DenseMapPair its own members instead of a std::pair base. NFC (PR #221853)

[Fangrui Song via llvm-commits]

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@@ -606,7 +606,10 @@ GVNSink::analyzeInstructionForSinking(LockstepReverseIterator<false> &LRI,
       return std::nullopt;
     VNums[N]++;
   }
-  unsigned VNumToSink = llvm::max_element(VNums, llvm::less_second())->first;
+  unsigned VNumToSink =
+      llvm::max_element(VNums, [](const auto &L, const auto &R) {
+        return L.second < R.second;
+      })->first;
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MaskRay wrote:

std::get can't be overloaded here: `[namespace.std]` allows only specializations of standard templates for program-defined types, and std::get is a function template, so adding overloads in namespace std is undefined behavior. 

https://github.com/llvm/llvm-project/pull/221853