LoongArch: rework copysign and xorsign implementation

The copysign and xorsign implementation had two significant bugs:

1. The GCC Internal documentation explicitly says the IOR, XOR, and AND
   optabs are only for fixed-point modes, i.e. they cannot be used for
   floating-point modes.
2. The handling of "%V" uses a very nasty way to pun floating-point
   const value to integer representation, invoking undefined behavior on
   32-bit hosts by shifting left a "long" by 32 bits.  In fact
   lowpart_subreg handles punning of const values correctly despite the
   name contains "reg."

Fix the bugs by using lowpart_subreg to pun the modes in the expanders.

gcc/

	* config/loongarch/predicates.md (const_vector_neg_fp_operand):
	New define_predicate.
	(reg_or_vector_neg_fp_operand): New define_predicate.
	* config/loongarch/lasx.md (copysign<mode>3): Remove.
	(xorsign<mode>3): Remove.
	* config/loongarch/lsx.md (copysign<mode>3): Remove.
	(@xorsign<mode>3): Remove.
	* config/loongarch/simd.md (copysign<mode>3): New define_expand.
	(@xorsign<mode>3): New define_expand.
	(and<mode>3): Only allow IVEC instead of ALLVEC.
	(ior<mode>3): Likewise.
	(xor<mode>3): Likewise.
	* config/loongarch/loongarch.cc (loongarch_print_operand): No
	longer allow floating-point vector constants for %V.
	(loongarch_const_vector_bitimm_set_p): Always return false for
	floating-point vector constants.
	(loongarch_build_signbit_mask): Factor out force_reg.
	(loongarch_emit_swrsqrtsf): Use integer vector mode instead of
	floating-point vector mode when masking zero inputs.

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