ada: Fix another minor fallout of previous changes to aggregate expansion

The processing of static array aggregates in Exp_Aggr requires that their
bounds be representable as Int(eger) values for practical purposes, and
the previous changes have exposed another path where this is not checked.

This introduces a UI_Are_In_Int_Range local predicate for convenience.

gcc/ada/ChangeLog:

	* exp_aggr.adb (UI_Are_In_Int_Range): New predicate.
	(Aggr_Size_OK): Use it.
	(Flatten): Likewise.
	(Packed_Array_Aggregate_Handled): Likewise.
	(Static_Array_Aggregate): Likewise.