[CodeLayout] Update to resolve Wdangling warning.

Change cc2fbc6 introduced -Wdangling
warning, use temporaries to resolve.

llvm/lib/Transforms/Utils/CodeLayout.cpp:764:27: error: temporary whose address is used as value of local variable '[minDensity, maxDensity]' will be destroyed at the end of the full-expression [-Werror,-Wdangling]
  764 |               std::minmax(ChainPred->density(), ChainSucc->density());

llvm/lib/Transforms/Utils/CodeLayout.cpp:764:49: error: temporary whose address is used as value of local variable '[minDensity, maxDensity]' will be destroyed at the end of the full-expression [-Werror,-Wdangling]
  764 |               std::minmax(ChainPred->density(), ChainSucc->density());

Author: alinas

llvm/lib/Transforms/Utils/CodeLayout.cpp

@@ -760,8 +760,10 @@ class ExtTSPImpl {
           // Skip the merge if the ratio between the densities exceeds
           // MaxMergeDensityRatio. Smaller values of the option result in fewer
           // merges, and hence, more chains.
+          auto ChainPredDensity = ChainPred->density();
+          auto ChainSuccDensity = ChainSucc->density();
           auto [minDensity, maxDensity] =
-              std::minmax(ChainPred->density(), ChainSucc->density());
+              std::minmax(ChainPredDensity, ChainSuccDensity);
           assert(minDensity > 0.0 && maxDensity > 0.0 &&
                  "incorrectly computed chain densities");
           const double Ratio = maxDensity / minDensity;