[AArch64][GISel] Add FP16 fcmp lowering

This adds v4f16 and v8f16 lowering for fp16 vector compares. It splits the
getActionDefinitionsBuilder of G_FCMP from G_ICMP, as they are quite different
operations, and adds fp16 vector lowering.

Differential Revision: https://reviews.llvm.org/D147947