x86: correct / simplify @vec_extract_hi_<mode> and vec_extract_hi_v32qi

x86: correct / simplify @vec_extract_hi_<mode> and vec_extract_hi_v32qi

The middle alternative each was unusable without enabling AVX512DQ (in
addition to AVX512VL), which is entirely unrelated here. The last
alternative is usable with AVX512VL only (due to type restrictions on
what may be put in the upper 16 YMM registers), and hence is pointlessly
forcing 512-bit mode (without actually reflecting that in the "mode"
attribute).

gcc/

	* config/i386/sse.md (@vec_extract_hi_<mode>): Drop last
	alternative. Switch new last alternative's "isa" attribute to
	"avx512vl".
	(vec_extract_hi_v32qi): Likewise.