[Ada] Streamline implementation of Has_Compatible_Representation

[Ada] Streamline implementation of Has_Compatible_Representation

The predicate is only concerned with the internal representation of types
and this representation is shared by the subtypes of a given type, so the
implementation can directly look into the (implementation) base types.

No functional changes.

gcc/ada/

	* sem_ch13.ads (Has_Compatible_Representation): Minor tweaks.
	* sem_ch13.adb (Has_Compatible_Representation): Look directly into
	the (implementation) base types and simplifiy accordingly.
	* exp_ch5.adb (Change_Of_Representation): Adjust.
	* exp_ch6.adb (Expand_Actuals): Likewise.