AArch64: disallow "fmov sD, #-0.0" during assembly.

We weren't checking the sign of the floating point immediate before translating
it to "fmov sD, wzr". Similarly for D-regs.

Technically "movi vD.2s, #0x80, lsl #24" would work most of the time, but it's
not a blessed alias (and I don't think it should be since people expect writing
sD to zero out the high lanes, and there's no dD equivalent). So an error it is.

rdar://20455398

llvm-svn: 234372