[Ada] Minor tweaks to dispatching support code

[Ada] Minor tweaks to dispatching support code

No functional changes.

gcc/ada/

	* exp_disp.ads (Expand_Interface_Thunk): Change type of Prim.
	* exp_disp.adb (Expand_Interface_Thunk): Declare Is_Predef_Op
	earlier, do not initialize Iface_Formal, use No idiom and tweaks
	comments.
	(Register_Primitive): Declare L earlier and tweak comments.
	* sem_disp.adb (Check_Dispatching_Operation): Move tests out of
	loop.