i386: Refactor ssedoublemode

ssedoublemode's double should mean double type, like SI -> DI.
And we need to refactor some patterns with <ssedoublemode> instead of
<ssedoublevecmode>.

gcc/ChangeLog:

	* config/i386/sse.md (ssedoublemode): Remove mappings to twice
	the number of same-sized elements. Add mappings to the same
	number of double-sized elements.
	(define_split for vec_concat_minus_plus): Change mode_attr from
	ssedoublemode to ssedoublevecmode.
	(define_split for vec_concat_plus_minus): Ditto.
	(<mask_codefor>avx512dq_shuf_<shuffletype>64x2_1<mask_name>):
	Ditto.
	(avx512f_shuf_<shuffletype>64x2_1<mask_name>): Ditto.
	(avx512vl_shuf_<shuffletype>32x4_1<mask_name>): Ditto.
	(avx512f_shuf_<shuffletype>32x4_1<mask_name>): Ditto.