ada: Fix interaction of aspect Predicate and static case expressions

The semantics of the GNAT-specific Predicate aspect should be equivalent
to those of the Static_Predicate aspect when the predicate expression is
static, but that is not correctly implemented for static case expressions.

gcc/ada/ChangeLog:

	* exp_ch4.adb (Expand_N_Case_Expression): Remove the test on
	enclosing predicate function for the return optimization.
	Rewrite it in the general case to catch all nondynamic predicates.
	(Expand_N_If_Expression): Remove the test on enclosing predicate
	function for the return optimization.