Teach the AArch64 backend about v4f16 and v8f16

This teaches the AArch64 backend to deal with the operations required
to deal with the operations on v4f16 and v8f16 which are exposed by
NEON intrinsics, plus the add, sub, mul and div operations.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216555 91177308-0d34-0410-b5e6-96231b3b80d8