[NVPTX] Lower nvvm.fmax to maximumnum not maxnum (#189976)
Converting nvvm.{fmin/fmax} into llvm.{min/max}num is slightly
incorrect, as {min/max}(a, sNaN) should produce "a" according to the PTX
spec, but LLVM's {min/max}num intrinsics may return either NaN or "a".
Use the {min/max}imumnum intrinsics instead for correct sNaN behaviour.
Also tidy up NVVM FMin/FMax constant-folding using these tighter
definitions of how the NVVM intrinsics map to {min/max}imum and
{min/max}imumnum.
