Teach the AArch64 backend to handle f16

This allows the AArch64 backend to handle fadd, fsub, fmul and fdiv
operations on f16 (half-precision) types by promoting to f32.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215891 91177308-0d34-0410-b5e6-96231b3b80d8