[AArch64] Fix handling of zero immediate in fmov instructions

Currently fmov #0 with a vector destination is handle incorrectly and results in
fmov #-1.9375 being emitted but should instead give an error. This is due to the
way we cope with fmov #0 with a scalar destination being an alias of fmov zr, so
fix this by actually doing it through an alias.

Differential Revision: https://reviews.llvm.org/D31949

llvm-svn: 300830