AVX512FP16: add truncmn2/extendmn2 expanders

AVX512FP16: add truncmn2/extendmn2 expanders

gcc/ChangeLog:

	* config/i386/sse.md (extend<ssePHmodelower><mode>2):
	New expander.
	(extendv4hf<mode>2): Likewise.
	(extendv2hfv2df2): Likewise.
	(trunc<mode><ssePHmodelower>2): Likewise.
	(avx512fp16_vcvt<castmode>2ph_<mode>): Rename to ...
	(trunc<mode>v4hf2): ... this, and drop constraints.
	(avx512fp16_vcvtpd2ph_v2df): Rename to ...
	(truncv2dfv2hf2): ... this, and likewise.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/avx512fp16-trunc-extendvnhf.c: New test.