[AArch64] Fix FCCMP with FP16 operands

Summary: This patch adds support for FCCMP instruction with FP16 operands, avoiding an assertion during instruction selection.

Reviewers: olista01, SjoerdMeijer, t.p.northover, javed.absar

Reviewed By: SjoerdMeijer

Subscribers: kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D50115

llvm-svn: 338554