[libc++] Optimize the two range overload of mismatch (#86853)

```
-----------------------------------------------------------------------------
Benchmark                                                 old             new
-----------------------------------------------------------------------------
bm_mismatch_two_range_overload<char>/1               0.941 ns         1.88 ns
bm_mismatch_two_range_overload<char>/2                1.43 ns         2.15 ns
bm_mismatch_two_range_overload<char>/3                1.95 ns         2.55 ns
bm_mismatch_two_range_overload<char>/4                2.58 ns         2.90 ns
bm_mismatch_two_range_overload<char>/5                3.75 ns         3.31 ns
bm_mismatch_two_range_overload<char>/6                5.00 ns         3.83 ns
bm_mismatch_two_range_overload<char>/7                5.59 ns         4.35 ns
bm_mismatch_two_range_overload<char>/8                6.37 ns         4.84 ns
bm_mismatch_two_range_overload<char>/16               11.8 ns         6.72 ns
bm_mismatch_two_range_overload<char>/64               45.5 ns         2.59 ns
bm_mismatch_two_range_overload<char>/512               366 ns         12.6 ns
bm_mismatch_two_range_overload<char>/4096             2890 ns         91.6 ns
bm_mismatch_two_range_overload<char>/32768           23038 ns          758 ns
bm_mismatch_two_range_overload<char>/262144         142813 ns         6573 ns
bm_mismatch_two_range_overload<char>/1048576        366679 ns        26710 ns
bm_mismatch_two_range_overload<short>/1              0.934 ns         1.88 ns
bm_mismatch_two_range_overload<short>/2               1.30 ns         2.58 ns
bm_mismatch_two_range_overload<short>/3               1.76 ns         3.28 ns
bm_mismatch_two_range_overload<short>/4               2.24 ns         3.98 ns
bm_mismatch_two_range_overload<short>/5               2.80 ns         4.92 ns
bm_mismatch_two_range_overload<short>/6               3.58 ns         6.01 ns
bm_mismatch_two_range_overload<short>/7               4.29 ns         7.03 ns
bm_mismatch_two_range_overload<short>/8               4.67 ns         7.39 ns
bm_mismatch_two_range_overload<short>/16              9.86 ns         13.1 ns
bm_mismatch_two_range_overload<short>/64              38.9 ns         4.55 ns
bm_mismatch_two_range_overload<short>/512              348 ns         27.7 ns
bm_mismatch_two_range_overload<short>/4096            2881 ns          225 ns
bm_mismatch_two_range_overload<short>/32768          23111 ns         1715 ns
bm_mismatch_two_range_overload<short>/262144        184846 ns        14416 ns
bm_mismatch_two_range_overload<short>/1048576       742885 ns        57264 ns
bm_mismatch_two_range_overload<int>/1                0.838 ns         1.19 ns
bm_mismatch_two_range_overload<int>/2                 1.19 ns         1.65 ns
bm_mismatch_two_range_overload<int>/3                 1.83 ns         2.06 ns
bm_mismatch_two_range_overload<int>/4                 2.38 ns         2.42 ns
bm_mismatch_two_range_overload<int>/5                 3.60 ns         2.47 ns
bm_mismatch_two_range_overload<int>/6                 3.68 ns         3.05 ns
bm_mismatch_two_range_overload<int>/7                 4.32 ns         3.36 ns
bm_mismatch_two_range_overload<int>/8                 5.18 ns         3.58 ns
bm_mismatch_two_range_overload<int>/16                10.6 ns         2.84 ns
bm_mismatch_two_range_overload<int>/64                39.0 ns         7.78 ns
bm_mismatch_two_range_overload<int>/512                247 ns         53.9 ns
bm_mismatch_two_range_overload<int>/4096              1927 ns          429 ns
bm_mismatch_two_range_overload<int>/32768            15569 ns         3393 ns
bm_mismatch_two_range_overload<int>/262144          125413 ns        28504 ns
bm_mismatch_two_range_overload<int>/1048576         504549 ns       112729 ns
```

1 files changed, 16 insertions, 0 deletions

diff --git a/libcxx/benchmarks/algorithms/mismatch.bench.cpp b/libcxx/benchmarks/algorithms/mismatch.bench.cpp
index 0628906..7917828 100644
--- a/libcxx/benchmarks/algorithms/mismatch.bench.cpp
+++ b/libcxx/benchmarks/algorithms/mismatch.bench.cpp
@@ -37,4 +37,20 @@ BENCHMARK(bm_mismatch<char>)->Apply(BenchmarkSizes);
 BENCHMARK(bm_mismatch<short>)->Apply(BenchmarkSizes);
 BENCHMARK(bm_mismatch<int>)->Apply(BenchmarkSizes);
 
+template <class T>
+static void bm_mismatch_two_range_overload(benchmark::State& state) {
+  std::vector<T> vec1(state.range(), '1');
+  std::vector<T> vec2(state.range(), '1');
+  std::mt19937_64 rng(std::random_device{}());
+
+  vec1.back() = '2';
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(vec1);
+    benchmark::DoNotOptimize(std::mismatch(vec1.begin(), vec1.end(), vec2.begin(), vec2.end()));
+  }
+}
+BENCHMARK(bm_mismatch_two_range_overload<char>)->DenseRange(1, 8)->Range(16, 1 << 20);
+BENCHMARK(bm_mismatch_two_range_overload<short>)->DenseRange(1, 8)->Range(16, 1 << 20);
+BENCHMARK(bm_mismatch_two_range_overload<int>)->DenseRange(1, 8)->Range(16, 1 << 20);
+
 BENCHMARK_MAIN();