ada: Create operator nodes in functional style

ada: Create operator nodes in functional style

A recent patch removed two rewritings, where we kept the operator node
but replaced its operands. This patch removes explicit setting of the
operands; instead, the operator is already created together with its
operands, which seems a bit safer and more consistent with how we
typically create operator nodes.

It is a cleanup only; semantics is unaffected.

gcc/ada/

	* exp_ch4.adb
	(Expand_Modular_Addition): Rewrite using Make_XXX calls.
	(Expand_Modular_Op): Likewise.
	(Expand_Modular_Subtraction): Likewise.
	* exp_imgv.adb
	(Expand_User_Defined_Enumeration_Image): Likewise.