Only allow (copysign x, NEG_CONST) -> (fneg (fabs x)) simplification for constant...

Only allow (copysign x, NEG_CONST) -> (fneg (fabs x)) simplification for constant folding [PR112483]

On targets with native copysign instructions, (copysign x, -1) is
usually more efficient than (fneg (fabs x)).  Since r14-5284, in the
middle end we always optimize (fneg (fabs x)) to (copysign x, -1), not
vice versa.  If the target does not support native fcopysign,
expand_COPYSIGN will expand it as (fneg (fabs x)) anyway.

gcc/ChangeLog:

	PR rtl-optimization/112483
	* simplify-rtx.cc (simplify_binary_operation_1) <case COPYSIGN>:
	Call simplify_unary_operation for NEG instead of
	simplify_gen_unary.