[AArch64] Use fneg instead of fsub -0.0, X Cin IR expansion of __builtin_neon_vfmsh_f16.

Addresses the FIXME and removes the only in tree use of
llvm::ConstantFP::getZeroValueForNegation for an FP type.

Reviewed By: dmgreen, SjoerdMeijer

Differential Revision: https://reviews.llvm.org/D147497