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AVX512FP16: Optimize _Float16 reciprocal for div and sqrt

Hongyu Wang requested to merge users/intel/hongyuw1/fp16_rcp_optimize into users/intel/fp16_rcp/master 
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-iteration.

gcc/ChangeLog:

	* 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<mask_scalar_name>): New define_insn
	for rpad pass.
	(*avx512fp16_vmrsqrtv8hf2<mask_scalar_name>): Likewise.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/avx512fp16-recip-1.c: New test.
	* gcc.target/i386/avx512fp16-recip-2.c: Ditto.

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