AArch64: implement efficient f16 bitcasts

Because i16 is illegal, there's no native DAG method to
represent a bitcast to or from an f16 type. This meant LLVM was
inserting a stack store/load pair which is really not ideal.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213378 91177308-0d34-0410-b5e6-96231b3b80d8