[AArch64] Legalize horizontal fmax/fmin reductions on f16 vectors

Expand the horizontal reduction during the instruction selection phase, but only if the target doesn't support the full fp16 instruction set.

Fixes https://bugs.llvm.org/show_bug.cgi?id=49401

Reviewed By: aemerson

Differential Revision: https://reviews.llvm.org/D97840