ada: Compiler goes into loop

ada: Compiler goes into loop

In some cases that are difficult to characterize, the compiler fails an
assertion check (if the compiler is built with assertions enabled) or
loops forever (if assertions are not enabled). One way this can happen is if
Exp_Util.Insert_Actions is called with an N_Itype_Reference node as its first
parameter. This, in turn, can happen when an instance of
Exp_Attr.Expand_N_Attribute_Reference.Built_And_Insert_Type_Attr_Subp
calls Insert_Action (which will call Insert_Actions).

gcc/ada/

	* exp_util.adb
	(Insert_Actions): Code was relying on an incorrect assumption that an
	N_Itype_Reference cannot occur in declaration list or a statement
	list.  Fix the code to handle this case.