x86/AVX512: use VMOVDDUP for broadcast to V2DF

x86/AVX512: use VMOVDDUP for broadcast to V2DF

Like is already the case for the AVX/AVX2 form, VMOVDDUP - acting on
double precision floating values - is more appropriate to use here, and
it can also result in shorter insn encodings when source is memory or
%xmm0...%xmm7, and no masking is applied (in allowing a 2-byte VEX
prefix then instead of a 3-byte one).

gcc/

	* config/i386/sse.md (<avx512>_vec_dup<mode><mask_name>): Use
	vmovddup.