i386: Implement VxHF vector set/insert/extract with lower ABI levels

i386: Implement VxHF vector set/insert/extract with lower ABI levels

This is a preparation patch that moves VxHF vector set/insert/extract
expansions from AVX512FP16 ABI to lower ABIs.  There are no functional
changes for -mavx512fp16 and a follow-up patch is needed to actually
enable VxHF vector modes for lower ABIs.

2021-12-14  Uroš Bizjak  <ubizjak@gmail.com>

gcc/ChangeLog:

	PR target/103571
	* config/i386/i386-expand.c (ix86_expand_vector_init_duplicate)
	<case E_V8HFmode>: Implement for TARGET_SSE2.
	<case E_V16HFmode>: Implement for TARGET_AVX.
	<case E_V32HFmode>: Implement for TARGET_AVX512F.
	(ix86_expand_vector_set_var): Handle V32HFmode
	without TARGET_AVX512BW.
	(ix86_expand_vector_extract)
	<case E_V8HFmode>: Implement for TARGET_SSE2.
	<case E_V16HFmode>: Implement for TARGET_AVX.
	<case E_V32HFmode>: Implement for TARGET_AVX512BW.
	(expand_vec_perm_broadcast_1) <case E_V8HFmode>: New.
	* config/i386/sse.md (VI12HF_AVX512VL): Remove
	TARGET_AVX512FP16 condition.
	(V): Ditto.
	(V_256_512): Ditto.
	(avx_vbroadcastf128_<mode>): Use V_256H mode iterator.