Fix (fcopysign x, NEGATIVE_CONST) -> (fneg (fabs x)) simplification [PR112483]

(fcopysign x, NEGATIVE_CONST) can be simplified to (fneg (fabs x)), but
a logic error in the code caused it mistakenly simplified to (fneg x)
instead.

gcc/ChangeLog:

	PR rtl-optimization/112483
	* simplify-rtx.cc (simplify_binary_operation_1) <case COPYSIGN>:
	Fix the simplification of (fcopysign x, NEGATIVE_CONST).