[AArch64] allow v8f16 types when FullFP16 is supported

This adds support for allowing v8f16 vector types, thus avoiding conversions
from/to single precision for these types. This is a follow up patch of
commits r311154 and r312104, which added support for scalars and v4f16
types, respectively.

Differential Revision: https://reviews.llvm.org/D37802

llvm-svn: 313351