[AArch64] Don't materialize 0 with "fmov h0, .." when FullFP16 is not supported

We were generating "fmov h0, wzr" instructions when FullFP16 is not enabled.
I've not added any tests, because the problem was visible in:
test/CodeGen/AArch64/arm64-zero-cycle-zeroing.ll,
which I had to change: I don't think Cyclone has FullFP16 enabled
by default, so it shouldn't be using this v8.2a instruction.

I've also removed these rdar tags, please shout if there are any objections.

Differential Revision: https://reviews.llvm.org/D43020

llvm-svn: 324581