MIPS: Fix wrong MSA FP vector negation

We expanded (neg x) to (minus const0 x) for MSA 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 MSA enabled.

Use the bnegi.df instructions to simply reverse the sign bit instead.

gcc/ChangeLog:

	* config/mips/mips-msa.md (elmsgnbit): New define_mode_attr.
	(neg<mode>2): Change the mode iterator from MSA to IMSA because
	in FP arithmetic we cannot use (0 - x) for -x.
	(neg<mode>2): New define_insn to implement FP vector negation,
	using a bnegi instruction to negate the sign bit.