Optimize _Float16 usage for non AVX512FP16.

1. No memory is needed to move HI/HFmode between GPR and SSE registers
under TARGET_SSE2 and above, pinsrw/pextrw are used for them w/o
AVX512FP16.
2. Use gen_sse2_pinsrph/gen_vec_setv4sf_0 to replace
ix86_expand_vector_set in extendhfsf2/truncsfhf2 so that redundant
initialization cound be eliminated.

gcc/ChangeLog:

	PR target/102811
	* config/i386/i386.c (inline_secondary_memory_needed): HImode
	move between GPR and SSE registers is supported under
	TARGET_SSE2 and above.
	* config/i386/i386.md (extendhfsf2): Optimize expander.
	(truncsfhf2): Ditto.
	* config/i386/sse.md (sse2p4_1): Adjust attr for V8HFmode to
	align with V8HImode.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/pr102811-2.c: New test.
	* gcc.target/i386/avx512vl-vcvtps2ph-pr102811.c: Add new
	scan-assembler-times.