[AArch64] Promote f16 operations to f32.

For the most common ones (such as fadd), we already did the promotion.
Do the same thing for all the others.

Currently, we'll just crash/assert on all these operations, as
there's no hardware or libcall support whatsoever.

f16 (half) is specified as an interchange - not arithmetic - format,
and is expected to be promoted to single-precision for arithmetic
operations.

While there, teach the legalizer about promoting some of the (mostly
floating-point) operations that we never needed before.

Differential Revision: http://reviews.llvm.org/D8648
See related discussion on the thread for: http://reviews.llvm.org/D8755

llvm-svn: 234550