[AArch64][GlobalISel] Legalize scalar G_FMINNUM + G_FMAXNUM

For subtargets with full FP16, this is legal for s16, s32, and s64. Without
full FP16, it's legal for s32 and s64.

For s128, this is a libcall.

We also support some vector types, but for now, let's just support scalars.

Differential Revision: https://reviews.llvm.org/D108259