AVX512FP16: Add fix(uns)?_truncmn2 for HF scalar and vector modes

AVX512FP16: Add fix(uns)?_truncmn2 for HF scalar and vector modes

NB: 64bit/32bit vectorize for HFmode is not supported for now, will
adjust this patch when V2HF/V4HF operations supported.

gcc/ChangeLog:

	* config/i386/i386.md (fix<fixunssuffix>_trunchf<mode>2): New expander.
	(fixuns_trunchfhi2): Likewise.
	(*fixuns_trunchfsi2zext): New define_insn.
	* config/i386/sse.md (ssePHmodelower): New mode_attr.
	(fix<fixunssuffix>_trunc<ssePHmodelower><mode>2):
	New expander for same element vector fix_truncate.
	(fix<fixunssuffix>_trunc<ssePHmodelower><mode>2):
	Likewise for V4HF to V4SI/V4DI fix_truncate.
	(fix<fixunssuffix>_truncv2hfv2di2):
	Likeise for V2HF to V2DI fix_truncate.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/avx512fp16-trunchf.c: New test.
	* gcc.target/i386/avx512fp16-truncvnhf.c: Ditto.