sign(float64<N>) generates incorrect code on NEON64 using gcc 9.3.0 with -ffast-math

[peabody-korg]

auto foo24(float64x2 a) { return sign(a); }

generates:

0000000000000000 <Inspiration::foo24(Inspiration::FloatVectorBase<simdpp::arch_neonfltsp::float64<4u, void> >)>:
   0:	6f00e402 	movi	v2.2d, #0x0
   4:	d10383ff 	sub	sp, sp, #0xe0
   8:	910383ff 	add	sp, sp, #0xe0
   c:	4e221c00 	and	v0.16b, v0.16b, v2.16b
  10:	4e221c21 	and	v1.16b, v1.16b, v2.16b
  14:	d65f03c0 	ret

The problem seems to trace back to the way bit_and() is being processed in the implementation of _i_sign(float64):

    return bit_and(a, 0x8000000000000000);

The 0x8000000'00000000 constant gets bit-converted to the floating point constant -0.0. Because of -ffast-math -0.0 and 0.0 are equivalent, so this winds up masking with 0 instead.