AVX512FP16: Optimize _Float16 reciprocal for div and sqrt

AVX512FP16: Optimize _Float16 reciprocal for div and sqrt

For _Float16 type, add insn and expanders to optimize x / y to
x * rcp (y), and x / sqrt (y) to x * rsqrt (y).
As Half float only have minor precision difference between div and
mul * rcp, there is no need for Newton-Rhapson approximation.

gcc/ChangeLog:

	* config/i386/i386.c (use_rsqrt_p): Add mode parameter, enable
	  HFmode rsqrt without TARGET_SSE_MATH.
	(ix86_optab_supported_p): Refactor rint, adjust floor, ceil,
	btrunc condition to be restricted by -ftrapping-math, adjust
	use_rsqrt_p function call.
	* config/i386/i386.md (rcphf2): New define_insn.
	(rsqrthf2): Likewise.
	* config/i386/sse.md (div<mode>3): Change VF2H to VF2.
	(div<mode>3): New expander for HF mode.
	(rsqrt<mode>2): Likewise.
	(*avx512fp16_vmrcpv8hf2): New define_insn for rpad pass.
	(*avx512fp16_vmrsqrtv8hf2): Likewise.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/avx512fp16-recip-1.c: New test.
	* gcc.target/i386/avx512fp16-recip-2.c: Ditto.
	* gcc.target/i386/pr102464.c: Add -fno-trapping-math.