[AArch64] added FP16 vcvth intrinsic support

Summary: Change-Id: I0df845749c7689dfc99150ba7c19c7d0dadbd705

Reviewers: javed.absar, SjoerdMeijer

Reviewed By: SjoerdMeijer

Subscribers: llvm-commits, SjoerdMeijer

Differential Revision: https://reviews.llvm.org/D46311

llvm-svn: 333410