[SCEV] Fix -Wrange-loop-construct (NFC)

/llvm-project/llvm/lib/Analysis/ScalarEvolution.cpp:12009:21:
 error: loop variable '[S, Mul]' creates a copy from type 'const value_type' (aka 'const llvm::detail::DenseMapPair<const llvm::SCEV *, int>') [-Werror,-Wrange-loop-construct]
    for (const auto [S, Mul] : Multiplicity) {
                    ^
/llvm-project/llvm/lib/Analysis/ScalarEvolution.cpp:12009:10:
 note: use reference type 'const value_type &' (aka 'const llvm::detail::DenseMapPair<const llvm::SCEV *, int> &') to prevent copying
    for (const auto [S, Mul] : Multiplicity) {
         ^~~~~~~~~~~~~~~~~~~~~
                    &

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -12006,7 +12006,7 @@ ScalarEvolution::computeConstantDifference(const SCEV *More, const SCEV *Less) {
     // Check whether all the non-constants cancel out, or reduce to new
     // More/Less values.
     const SCEV *NewMore = nullptr, *NewLess = nullptr;
-    for (const auto [S, Mul] : Multiplicity) {
+    for (const auto &[S, Mul] : Multiplicity) {
       if (Mul == 0)
         continue;
       if (Mul == 1) {