[AArch64] Always lower fp16 zero to FMOVH0

We can always use FMOVH0 to lower fp16 zero, even without fullfp16. We can
either expand it to movi d0, #0 or fmov s0, wzr, which will both clear all the
bits of the register.

Differential Revision: https://reviews.llvm.org/D143988