spirv: preserve signed zero in modf
fsign's result can be +0.0 or -0.0 for -0.0. We already calculate the signed zero, it's even faster to replace the fmul(fsign(x), ...) with ior.
fsign's result can be +0.0 or -0.0 for -0.0. We already calculate the signed zero, it's even faster to replace the fmul(fsign(x), ...) with ior.