[AArch64] Add FMOVH0: materialize 0 using zero register for f16 values

Instead of loading 0 from a constant pool, it's of course much better to
materialize it using an fmov and the zero register.

Thanks to Ahmed Bougacha for the suggestion.

Differential Revision: https://reviews.llvm.org/D37102

llvm-svn: 311662