Optimize fmax with NAN

Fp max reductions would typically look like:

  for(nvfuser_index_t i154 = 0; i154 < 8; ++i154) {
    int i299;
    i299 = 4 * i154;
#pragma unroll
      for(nvfuser_index_t i156 = 0; i156 < 4; ++i156) {
        T29[0] = fmax(
            T29[0],
            T24[(i299 + i156)]);
      }
  }

Here, fmax is is not just fmaxf, but it also incurs two more comparisons in case the arguments are NAN: https://github.com/NVIDIA/Fuser/blob/main/runtime/helpers.cu#LL102C1-L111C2

This could be translated as:

  bool is_nan = false;
#pragma unroll
  for(nvfuser_index_t i154 = 0; i154 < 8; ++i154) {
    int i299;
    i299 = 4 * i154;
#pragma unroll
      for(nvfuser_index_t i156 = 0; i156 < 4; ++i156) {
#if 0
        T29[0] = fmax(
            T29[0],
            T24[(i299 + i156)]);
#else
        T29[0] = T29[0] > T24[(i299 + i156)] ? T29[0] : T24[(i299 + i156)];
        is_nan = is_nan || isnan(T24[(i299 + i156)]);
#endif
      }
  }
  if (is_nan) {
    T29[0] = NAN;
  }

In the case of cross entropy loss (#278), I observed 20% speedup on A100.

I think this translation could be done automatically as part of lowering. See the translation for welford vectorization.