[AArch64] Do not promote f16 when subtarget HasFullFP16

Armv8.2-A adds FP16 support, i.e. f16 is not only a storage-only type, but it
also supports performing data processing on 16-bit floating-point quantities.
All the necessary (tablegen) groundwork of adding the ARMv8.2-A FP16 (scalar)
instructions was done in D15014. To take advantage of this, this patch avoids
promotion of f16 to f32 types when the subtarget supports FullFP16, which
enables instruction selection of these FP16 instructions.

Differential Revision: https://reviews.llvm.org/D36396

llvm-svn: 311154