You can get automatic vectorization (with -O3) like this:

    bool is_sorted(const int32_t* input, size_t n) {
      int32_t sorted = true;

      for (size_t i = 1; i < n; ++i) {
        sorted &= input[i - 1] <= input[i];
      }

      return sorted;
    }

And the performance is similar to the AVX version (benchmarked on a MacBook Air early 2015):

    $ ./benchmark_avx2 1048576
    input size 1048576, iterations 10
    scalar         : 6379 us
    SSE (generic)  : 3544 us
    SSE            : 3704 us
    My example     : 2769 us
    AVX2 (generic) : 2679 us
    AVX2           : 3360 us

So I'm getting 2769us with the above 5 simple lines of code. It's just 3% slower (that might be noise).