[mlir] Apply ClangTidy performance fix.

Use const reference for loop variable.

Author: akuegel

mlir/lib/Analysis/Presburger/Barvinok.cpp

@@ -425,7 +425,7 @@ mlir::presburger::detail::computePolytopeGeneratingFunction(
     // cones.
     GeneratingFunction vertexGf(numSymbols, {}, {}, {});
     SmallVector<std::pair<int, ConeH>, 4> unimodCones = {{1, tangentCone}};
-    for (std::pair<int, ConeH> signedCone : unimodCones) {
+    for (const std::pair<int, ConeH> &signedCone : unimodCones) {
       auto [sign, cone] = signedCone;
       vertexGf = vertexGf +
                  computeUnimodularConeGeneratingFunction(*vertex, sign, cone);
@@ -785,4 +785,4 @@ mlir::presburger::detail::computeNumTerms(const GeneratingFunction &gf) {
   }
 
   return totalTerm.simplify();
-}
+}