[Ada] Avoid creating temporaries in Universal_Integer for range checks

[Ada] Avoid creating temporaries in Universal_Integer for range checks

2020-06-03  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* checks.adb (Is_Single_Attribute_Reference): New predicate.
	(Generate_Range_Check): Do not force the evaluation if the
	node is a single attribute reference.
	* exp_util.adb (Side_Effect_Free_Attribute): New predicate.
	(Side_Effect_Free) <N_Attribute_Reference>: Call it.
	(Remove_Side_Effects): Remove the side effects of the prefix
	for an attribute reference whose prefix is not a name.