x86: drop stray "prefix_extra"

While the attribute is relevant for legacy- and VEX-encoded insns, it is
of no relevance for EVEX-encoded ones.

While there in <mask_codefor>avx512dq_broadcast<mode><mask_name>_1 add
the missing "length_immediate".

gcc/

	* config/i386/sse.md
	(*<avx512>_eq<mode>3<mask_scalar_merge_name>_1): Drop
	"prefix_extra".
	(avx512dq_vextract<shuffletype>64x2_1_mask): Likewise.
	(*avx512dq_vextract<shuffletype>64x2_1): Likewise.
	(avx512f_vextract<shuffletype>32x4_1_mask): Likewise.
	(*avx512f_vextract<shuffletype>32x4_1): Likewise.
	(vec_extract_lo_<mode>_mask [AVX512 forms]): Likewise.
	(vec_extract_lo_<mode> [AVX512 forms]): Likewise.
	(vec_extract_hi_<mode>_mask [AVX512 forms]): Likewise.
	(vec_extract_hi_<mode> [AVX512 forms]): Likewise.
	(@vec_extract_lo_<mode> [AVX512 forms]): Likewise.
	(@vec_extract_hi_<mode> [AVX512 forms]): Likewise.
	(vec_extract_lo_v64qi): Likewise.
	(vec_extract_hi_v64qi): Likewise.
	(*vec_widen_umult_even_v16si<mask_name>): Likewise.
	(*vec_widen_smult_even_v16si<mask_name>): Likewise.
	(*avx512f_<code><mode>3<mask_name>): Likewise.
	(*vec_extractv4ti): Likewise.
	(avx512bw_<code>v32qiv32hi2<mask_name>): Likewise.
	(<mask_codefor>avx512dq_broadcast<mode><mask_name>_1): Likewise.
	Add "length_immediate".