[Ada] Small cleanup throughout Exp_Ch4

[Ada] Small cleanup throughout Exp_Ch4

gcc/ada/

	* exp_ch4.adb (Expand_Array_Comparison): Reformat.
	(Expand_Concatenate): Use standard size values directly and use
	Standard_Long_Long_Unsigned instead of RE_Long_Long_Unsigned.
	(Expand_Modular_Op): Use Standard_Long_Long_Integer in case the
	modulus is larger than Integer.
	(Expand_N_Op_Expon): Use standard size value directly.
	(Narrow_Large_Operation): Use Uint instead of Nat for sizes and
	use a local variable for the size of the type.
	(Get_Size_For_Range): Return Uint instead of Nat.
	(Is_OK_For_Range): Take Uint instead of Nat.