re PR tree-optimization/85466 (Performance is slow when doing 'branchless' conditional style math operations)

	PR libstdc++/85466
	* real.h (real_nextafter): Declare.
	* real.c (real_nextafter): New function.
	* fold-const-call.c (fold_const_nextafter): New function.
	(fold_const_call_sss): Call it for CASE_CFN_NEXTAFTER and
	CASE_CFN_NEXTTOWARD.
	(fold_const_call_1): For CASE_CFN_NEXTTOWARD call fold_const_call_sss
	even when arg1_mode is different from arg0_mode.

	* gcc.dg/nextafter-1.c: New test.
	* gcc.dg/nextafter-2.c: New test.
	* gcc.dg/nextafter-3.c: New test.
	* gcc.dg/nextafter-4.c: New test.

From-SVN: r259921