Adjust hlsl infinity-constant test

[dneto0]

Test/hlsl.inf.vert tests parsing and some constant math on infinities, including (-1.#INF * 0.0). By IEEE 754 rules, that result is a NaN, but its sign is not significant. The test output assumes a negative-NaN is in the generated SPIR-V.

However, the math library on some platforms (like macOS 14, a.k.a. Sonoma) will produce a positive NaN instead.

This PR adjusts the test so it takes the absolute value of the NaN, to ensure we the emitted SPIR-V has the NaN with a 0 for it sign bit.

[dneto0]

I encountered this on an Apple Silicon mac with a recent Xcode.

I also replicated it on an x86-64 Linux machine with Clang, but not GCC.

Here's a short and messy program that shows what's going on:

#include <iostream>
#include <cstring>
#include <iomanip>

int main() {
  const float neg_inf = -1.0f/0.0f;
  const float nan = neg_inf * 0.0f;
  std::cout << " neg_inf " << neg_inf << std::endl;
  std::cout << " nan " << nan << std::endl;
  int i0;
  int i1;
  std::memcpy(&i0, &neg_inf, 4);
  std::memcpy(&i1, &nan, 4);
  std::cout << " i0 " << std::hex << i0 << std::endl;
  std::cout << " i1 " << std::hex << i1 << std::endl;
}

$ g++ -std=c++11 x.cpp -o x
$ ./x
 neg_inf -inf
 nan -nan
 i0 ff800000
 i1 ffc00000
$ clang++ -std=c++11 x.cpp -o x
$ ./x
 neg_inf -inf
 nan nan
 i0 ff800000
 i1 7fc00000

This is GCC 13.2.0, Clang 16.