[Ada] Fix oversight for case expression in Eval_Integer_Literal

[Ada] Fix oversight for case expression in Eval_Integer_Literal

The intent of the entry test is to treat conditional expressions, that is
to say if-expression and case-expression, alike and to require that a second
condition be true for them.  But an N_Case_Expression_Alternative is not an
N_Subexpr so this second condition was short-circuited for this node.

gcc/ada/

	* sem_eval.adb (Eval_Integer_Literal): Fix oversight in entry test.