LoongArch: Fix wrong LSX FP vector negation

We expanded (neg x) to (minus const0 x) for LSX FP vectors, this is
wrong because -0.0 is not 0 - 0.0.  This causes some Python tests to
fail when Python is built with LSX enabled.

Use the vbitrevi.{d/w} instructions to simply reverse the sign bit
instead.  We are already doing this for LASX and now we can unify them
into simd.md.

gcc/ChangeLog:

	* config/loongarch/lsx.md (neg<mode:FLSX>2): Remove the
	incorrect expand.
	* config/loongarch/simd.md (simdfmt_as_i): New define_mode_attr.
	(elmsgnbit): Likewise.
	(neg<mode:FVEC>2): New define_insn.
	* config/loongarch/lasx.md (negv4df2, negv8sf2): Remove as they
	are now instantiated in simd.md.