Support smin/smax for V2HF/V4HF

gcc/ChangeLog:

	* config/i386/mmx.md (VHF_32_64): New mode iterator.
	(<insn><mode>3): New define_expand, merged from ..
	(<insn>v4hf3): .. this and
	(<insn>v2hf3): .. this.
	(movd_v2hf_to_sse_reg): New define_expand, splitted from ..
	(movd_v2hf_to_sse): .. this.
	(<code><mode>3): New define_expand.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/part-vect-vminmaxph-1.c: New test.
	* gcc.target/i386/avx512fp16-64-32-vecop-1.c: Scan-assembler
	only for { target { ! ia32 } }.