Use LLVM isnan intrinsic to implement SLEIGH's FLOAT_NAN

[tetsuo-cpp]

As of #585, we're implementing FLOAT_NAN with our own code that checks the float against itself with an ordered float comparison.

LLVM is considering an isnan intrinsic that would be helpful for us. It got merged and then subsequently reverted in llvm/llvm-project@3f1f08f but it'd be good to keep an eye on this and see whether it makes its way into LLVM 15.